Teeth and Digestion

Feel the shapes of your teeth with your tongue. The front edges feel different from the broad back teeth. Each shape helps with a different part of eating.

Four kinds of teeth

You have eight incisors: four above and four below. There are four canines, one at each corner of the front teeth. A full adult set has eight premolars. Baby teeth include molars, but no premolars.

The little points on a premolar are called cusps. Most premolars have two main cusps. You may hear them called bicuspids. A molar has a larger chewing surface, with bumps and grooves. It can press food down and grind it as your jaws move.

Follow a bite of apple

First, your incisors cut a piece from the apple. Your canines help grip and tear its tough skin. Your tongue moves the piece toward your back teeth. Premolars crush it, and molars grind it into softer pieces. If you still have baby teeth, your baby molars do the back-tooth chewing.

Your tongue keeps moving the pieces between the chewing surfaces. Your cheeks help keep food inside your mouth. When the food is soft, your tongue gathers it into a lump for swallowing.

Match shape to job

Imagine trying to mash cooked potato with a thin edge. A wide, bumpy surface would press more of it at once. That helps explain why molars suit grinding and incisors suit cutting.

Canines help with chewy bread as well as meat. A tooth's job depends on its shape, not on one special food. For most chewing, food moves back toward the premolars and molars. Your front teeth start many bites, but they do not do most of the grinding.

A tooth has a hard outside and living parts inside. The piece you see is only part of the whole tooth.

Above and below the gums

The crown is the part above your gums. The root sits below the gums, held in your jawbone. Incisors usually have one root. Molars usually have two or three roots to help hold them firmly.

Your gums fit around the teeth and cover the jawbone. Healthy gums feel firm. Their natural color can be pink, brown, or darker. Color alone does not tell you whether gums are healthy. Healthy gums do not bleed with gentle brushing.

From the outside to the center

The crown wears a tough outer coat called enamel. It is the hardest substance in your body, even harder than bone. It protects the tooth when you bite and chew.

Below the enamel is dentin, a yellowish layer. It is softer than enamel and contains tiny tubes. If enamel becomes thin or damaged, hot, cold, or sweet foods can affect the inside through these tubes. The tooth may feel sensitive.

The soft center is the pulp. It holds blood vessels and nerves. Blood vessels bring what the living tooth needs. Nerves let you feel heat, cold, and pain.

Think of a castle: enamel is its hard outer wall. Dentin is the layer behind that wall. Pulp is the room holding the living parts. The crown and root describe where parts of a tooth are. Enamel, dentin, and pulp describe what lies from outside to inside.

Why damage can hurt

Imagine decay making a small weak spot in the enamel. There may be no pain yet. As damage reaches deeper layers, the tooth can become sensitive. If decay reaches the pulp, it can cause a bad toothache. A painless tooth can still need a dentist's care.

Teeth are not bones, even though both contain calcium. A broken bone can heal. Your body cannot grow a fresh layer of enamel after it is lost. Keeping the enamel you have matters.

A loose tooth is part of growing up. Your mouth changes as adult teeth arrive and your jaw grows.

Your first set

Your first teeth are baby teeth, also called primary teeth. They start coming through at about six months. Most children have all 20 by about age three.

Baby teeth help you chew and speak clearly. They also hold spaces for adult teeth. If decay makes a baby tooth fall out too early, nearby teeth can move into its space. The adult tooth may then have less room.

What changes around age six?

Baby teeth usually start falling out around age six. Often, the bottom front incisors go first. A growing adult tooth moves upward beneath a baby tooth. The baby root slowly dissolves, the tooth loosens, and it falls out.

The first adult molars also arrive around this age. They grow behind the baby molars. No baby tooth falls out to make their spaces. These are often called six-year molars.

From about ages six to twelve, you have a mix of both sets. Most children lose their last baby teeth by about twelve or thirteen.

Compare the sets

Adult premolars replace baby molars. Adult molars grow farther back as the jaw has more room.

To check the totals, split the mouth into four quarters. Each baby quarter has two incisors, one canine, and two molars. That is five teeth in each quarter, making 20. Each full adult quarter has two incisors, one canine, two premolars, and three molars. Eight in each quarter makes 32.

The last adult molars

The last four molars are wisdom teeth. They usually arrive around ages seventeen to twenty-one. Some people never get some or all of them. Many adults therefore have 28 teeth instead of the full 32.

An adult tooth has no new replacement waiting underneath. Your adult teeth need care throughout your life.

A horse, a wolf, and a person eat different foods. Their teeth give clues about how they handle those foods.

Compare three groups

Grass and leaves can be tough. Horses, cows, sheep, and deer grind plant food with wide back teeth. Their canines are often small or missing. Many have a gap between the front and back teeth.

Cows and sheep have no upper front teeth. They press grass between their lower incisors and a hard upper pad. Then they pull the grass off. Horses have upper and lower incisors for nipping plants.

A wolf grips food with long, pointed canines. Sharp edges on its back teeth slice meat, like scissor blades. A cat has teeth suited to the same kind of work, even though it is much smaller.

Humans, pigs, and bears are omnivores. A mix of teeth helps them handle different foods. Human canines are much smaller than a wolf's.

Read the clues together

Suppose a skull has broad, ridged molars and very small canines. Grinding tough plants fits those shapes, so a plant-eater is a likely match. Now imagine long canines together with blade-like back teeth. Gripping and slicing meat fits that second set.

Use several tooth clues together. Body size is less useful: an elephant is a large plant-eater, while a house cat is a small meat-eater.

Teeth with special jobs

Beavers and squirrels have front teeth that keep growing. Gnawing wears those teeth down. Sharks lose teeth and grow replacements throughout life. An elephant's tusks are very long incisors.

Birds have no teeth. Many swallow small stones that help a muscular stomach part, the gizzard, grind food.

Teeth can look clean while a sticky film covers them. This film, called plaque, contains bacteria. It gathers between teeth, in back-tooth grooves, and along the gum line.

From food to a cavity

Bacteria are tiny living things in your mouth. Some plaque bacteria feed on sugar and starch left from food. They make acid as they feed.

1. You eat or drink food containing sugar or starch.
2. Plaque bacteria use it and make acid.
3. Acid takes minerals from the enamel and weakens it.
4. Repeated attacks can make a hole called a cavity.

The damage is tooth decay. Sugar does not drill a hole by itself. The bacteria and the acid explain how the damage happens.

The timing matters

A sugary snack starts an acid attack that lasts for a while. Sipping a sweet drink again and again keeps restarting the attack. Your teeth get less time to recover between sips.

For example, Sam spreads five small sweet snacks through the day. Mia eats the same sweets just after lunch. Sam has five separate sugary snack times. Mia has one. Eating the same amount at mealtime gives teeth a longer break afterward.

Sticky gummies and dried fruit can cling to teeth. Soda and fruit juice also contain acid themselves. That acid can wear enamel without bacteria making it first. Both the food you choose and how often you have it matter.

Early spots and deeper holes

A white or brown spot can be a sign of decay. Sweet, hot, or cold foods may make a tooth hurt. Sometimes early decay causes no pain, so a dentist checks before you notice a problem.

Fluoride can help strengthen an early weak spot before a hole forms. A cavity cannot close itself. A dentist removes the damaged material and repairs the hole with a filling.

Plaque can harm gums too

Plaque at the gum line can make gums sore, swollen, or more red than usual. They may bleed when brushed. This early gum disease is gingivitis. Gentle brushing and daily cleaning between teeth help remove the cause. Keep brushing carefully; tell an adult and your dentist if bleeding continues.

An eggshell can help you investigate acid damage. It is hard, contains calcium, and can soften in acid. Those features make it useful as a model of enamel.

Ask a question and predict

Try this question: "How do different liquids affect an eggshell?" Before starting, write a prediction, such as "I think vinegar will soften the shell because it is acidic."

An adult can help prepare five similar hard-boiled eggs. Use five matching cups with 200 milliliters (about 7 fluid ounces) of liquid in each. Compare water, milk, orange juice, cola, and vinegar. Vinegar is an acidic test liquid, not a drink to sip.

Leave one egg in each cup for three days, or 72 hours. Wash your hands after handling the eggs, and never eat the test eggs.

Make the test fair

Change the type of liquid. Keep the egg size and kind, liquid amount, cups, place, and soaking time the same. Keeping the cups together helps keep their temperature alike.

The water cup is the control. It gives you a comparison with plain water. Without it, you might mistake a change caused by soaking for a special effect of another liquid.

Record what happens

Look at the shells before soaking and again afterward. Record color, shell feel, and bubbles in a table. Gently compare whether shells feel hard, rough, or soft.

Add rows for the other liquids. Water and milk often leave shells looking similar to the start. Vinegar often makes bubbles and softens or dissolves the shell. Cola may stain it brown. Juice and cola often roughen or soften it too.

Explain the evidence

A useful conclusion links a result to the question: "The vinegar shell became soft, while the water shell stayed hard. Acid damaged the shell in this test."

A stain alone shows a color change. Check shell texture before describing damage. Your observations may differ from your prediction.

An eggshell has no plaque bacteria and is made from a different material than enamel. The test leaves out saliva, brushing, and bacteria turning sugar into acid. Teeth also meet drinks for far less time than this.

Plaque starts building up again after brushing. Caring for your teeth every day helps protect the enamel and gums, even when your teeth look clean.

Brush twice, for two minutes

Brush in the morning and before bed with fluoride toothpaste. Fluoride helps enamel stay strong against acid. Use a pea-sized amount. Spit it out after brushing instead of swallowing it.

Use a soft-bristled toothbrush. Tilt it toward the gum line and move it gently in short strokes or small circles. Clean the outer sides, inner sides, and chewing tops of every tooth. Brush your tongue gently too. Hard scrubbing can hurt the gums.

Two minutes is 120 seconds. Split your mouth into four quarters and spend about 30 seconds on each. A timer can help you finish all four without rushing.

Reach the gaps

Clean between teeth once each day with floss or a cleaner made for the gaps. A brush misses tight spaces where teeth touch. Ask an adult for help if those gaps are hard to reach.

Replace a toothbrush every three to four months, or sooner if its bristles become frayed. Do not share toothbrushes.

Choose drinks and snacks

Drink water between meals. Keep sweet foods and drinks mostly for mealtimes. Cheese, plain yogurt, vegetables, and whole fruit make useful snack choices. Slowly sipping soda keeps sugar and acid meeting your teeth over and over.

Some tap water contains added fluoride, and some does not. Your dentist can tell you whether you need extra fluoride.

Let the dentist help

Go for checkups as often as your dentist recommends. For many children, this is about every six months. The dentist looks for early decay and removes tartar, plaque that has hardened. Brushing cannot remove tartar.

A dentist can put sealants on the chewing tops of back teeth. These thin coatings help keep food and bacteria out of deep grooves.

Protect teeth during play

A mouthguard cushions your teeth during sports where your mouth could get hit. Football, hockey, basketball, and skateboarding are examples. If an adult tooth is knocked out, tell an adult right away. You need a dentist quickly.

Your body uses food to move, grow, and repair itself. But a piece of your lunch cannot pass straight into your blood. It first needs to be broken down.

What digestion does

Food contains nutrients. These include carbohydrates, proteins, fats, vitamins, and minerals. You also need water. Digestion breaks food down into small, simple substances your body can take in and use.

You break food down in two ways:

• Physical breakdown changes food into smaller pieces. Teeth chew it, and stomach muscles churn it.
• Chemical breakdown uses digestive juices to change food into simpler substances.

Imagine tearing a piece of bread into smaller crumbs. That shows physical breakdown: the pieces are still bread. In your body, digestive juices also change food in ways that tearing alone cannot do.

Chewing gives the juices more surface to work on. A big lump has much of its food tucked inside. Smaller pieces expose more food to the juices, helping digestion along.

Organs work as a team

An organ is a body part with a particular job. Your stomach is an organ. So is your heart. Organs that work together make an organ system.

Your digestive system breaks down food, takes in nutrients, and removes leftover waste. Its main pathway is the digestive tract, or gut. This is a long tube from the mouth to the anus. In an adult, it is about 9 meters (30 feet) long.

Helpers beside the tube

Some digestive organs sit beside the food pathway. They send useful liquids into it. Salivary glands make saliva for the mouth. The liver makes bile, and the gallbladder stores it. The pancreas makes digestive juice.

Food travels along the tract while these helpers add their liquids. It does not travel through the helper organs themselves.

The stomach does only part of the work. Most nutrients pass into your blood from the small intestine after food has been broken down. Blood then carries those nutrients to the parts of your body that need them.

Your digestive organs share the space inside your body. Knowing their positions helps you label a picture and follow the food pathway.

Follow the connected tube

Food passes through these parts in order:

Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus.

Your mouth contains teeth and a tongue. The esophagus, or food pipe, runs from the throat down through the chest. It sits behind the windpipe and reaches the stomach. In an adult, it is about 25 centimeters (10 inches) long.

The stomach is a stretchy, J-shaped bag. It sits high on your left side, partly under the lower ribs. It is higher than your belly button.

Two intestines, two shapes

The small intestine coils in the middle of your belly. It is narrow but very long: about 6 meters (20 feet) in an adult.

The large intestine is wider and shorter: about 1.5 meters (5 feet) in an adult. It frames the small intestine. It runs up your right side, across the top, then down your left side. A small pouch called the appendix hangs near its start, low on the right.

The rectum is the last part of the large intestine. Waste waits there before leaving through the anus, the opening at the end.

"Small" and "large" describe the tubes' widths. They do not describe their lengths. The small intestine is the longest part of the pathway.

Place the helpers

The liver, your largest internal organ, sits high on your right side. The little gallbladder is tucked underneath it. The pancreas lies behind the stomach. These helpers connect to the tract, but food does not pass through them.

Label a person facing you

On a front-view body picture, the person's left is on your right. Start by marking the person's left and right beside the drawing. Then place the stomach high on the person's left and the liver high on their right.

Next, trace the esophagus into the stomach. Find the narrow coils, then the wider frame around them. Finish with the rectum and anus below. This lets you check the connections as well as the labels.

Digestion starts in your mouth. Teeth, saliva, and your tongue work together before food reaches your stomach.

Make the food soft

Your teeth cut, tear, crush, and grind food. Saliva, or spit, wets the pieces so they become soft and slippery. It also starts breaking down starchy foods such as rice and bread.

Salivary glands make this liquid. They sit in your cheeks, under your tongue, and under your jaw. Seeing or smelling tasty food can make them produce extra saliva. That is why your mouth can feel as if it is "watering."

Your tongue moves food between your teeth and mixes in saliva. It helps you taste the food too. Then it gathers the chewed food into a soft lump called a bolus. The tongue pushes this toward the back of your throat.

Food and air use different tubes

Your windpipe, or trachea, carries air toward your lungs. Your esophagus carries swallowed food toward your stomach.

When you swallow, a small flap called the epiglottis covers the windpipe opening. Food moves toward the esophagus. If food starts going the wrong way, coughing helps push it out.

Muscles move the bite

Waves of squeezing muscle push the bolus through the esophagus. This movement is peristalsis. Think of squeezing toothpaste along a tube: a squeeze behind the paste moves it forward.

Gravity can help food move downward, but muscles do the pushing. This is why astronauts can swallow in space. Peristalsis also moves material along the stomach and intestines.

Follow a cracker step by step

First, incisors bite a piece from the cracker. Back teeth grind it into smaller bits. Saliva wets those bits, and the tongue rolls them together. The tongue pushes the bolus back, the epiglottis covers the windpipe, and peristalsis moves the bolus down the esophagus.

At the bottom, a ring of muscle opens to let food enter the stomach. It closes afterward. If stomach juices come back up, they can cause a burning feeling called heartburn. The name describes where it feels sore; it does not mean the heart is digesting food.

After swallowing, food reaches a stretchy bag high in your belly. This is your stomach. Its strong walls and digestive juices work on the meal together.

A stretchy storage bag

An empty stomach is small. As food enters, it stretches to hold the meal. The food stays there while muscles squeeze and mix it.

This mixing is called churning. Imagine gently squeezing a bag of wet food so the liquid reaches every part. Stomach muscles do a similar mixing job inside your body.

Juices help break food down

Stomach juices contain strong acid. The acid helps food break down, including protein foods such as eggs, beans, and meat. It also kills many germs swallowed with food and drink.

A thick, slippery layer of mucus coats the stomach's inner wall. This helps protect the wall from the acid. The muscles belong in the wall; the acid belongs inside the stomach with the food.

Handwashing still matters because stomach acid does not kill every germ.

From pieces to a soupy mixture

As churning and juices work, the meal becomes a thick, soupy mixture. This mixture is called chyme. A meal often spends about two to four hours in the stomach. A drink of water can leave faster than a rich meal containing lots of fat or protein.

For example, pieces of a bean burrito arrive after you chew and swallow. Stomach muscles fold and squeeze the pieces through the juices. The mixture becomes smoother. A muscle ring at the stomach's exit lets small amounts of chyme enter the small intestine.

Letting chyme out a little at a time gives the next part work it can handle. The whole meal does not rush out as one solid lump.

Digestion keeps going

The stomach does not finish the whole job. The small intestine continues breaking food down and takes in most nutrients. Your lunch has farther to travel after it leaves the stomach.

A rumbling belly comes from gas and liquid being moved by digestive muscles. It can happen when you are hungry or after eating. The sound is movement inside the tract.

The soupy chyme enters the small intestine after the stomach. More juices help finish breaking down the food. Then your body takes in the nutrients.

Three helpers add liquids

The pancreas sends digestive juice into the small intestine. This juice helps break down carbohydrates, proteins, and fats. The small intestine's walls make digestive juices too.

The liver makes a liquid called bile. The gallbladder stores bile and releases it into the small intestine. Bile breaks big blobs of fat into tiny droplets. Smaller droplets are easier for digestive juices to work on.

Think of separating one large blob into many little blobs. Bile helps with this first step; the other juices continue breaking the food down.

These liquids reach the intestine through small tubes called ducts. Food stays inside the digestive tract. It does not enter the liver, gallbladder, or pancreas.

Nutrients cross into the body

Digestion makes food substances small and simple enough to take in. Most nutrients pass through the wall of the small intestine into the blood. Taking substances into your body this way is called absorption.

Blood carries nutrients to body parts that need energy or materials to grow and repair. Your digestive system supplies nutrients, while your heart and blood vessels carry them around. The two systems work together.

Follow a cheese sandwich

First, chyme from the sandwich enters the small intestine. Bile helps split fat from the cheese into tiny droplets. Pancreas juice helps break down starch from the bread and protein from the cheese.

As the food becomes small, simple nutrients, these pass across the intestine's wall into the blood. Much of the fiber from whole-wheat bread stays in the tract. Your body cannot digest that fiber, so it travels onward with other leftovers.

Peristalsis keeps the mixture moving through the coils. In an adult, this narrow tube is about 6 meters (20 feet) long. The stomach and small intestine take several hours to work on a meal. Absorption happens along the journey, before the leftovers become feces.

After the small intestine, leftovers enter the large intestine. Much of what arrives is water and material your body has not digested, including fiber.

Take water back

The large intestine takes water from the leftovers back into your body. The leftovers become firmer as some water leaves them. They form feces, or poop.

Feces still contain plenty of water. They also contain fiber, bacteria, and old cells from the gut lining. They are not simply dry bits of yesterday's meal.

Helpful bacteria live here

Your large intestine is home to trillions of bacteria. Many help keep your gut healthy. They feed on fiber and other leftovers your body cannot break down. They also make some useful vitamins, including vitamin K.

Bacteria have different jobs in different places. Plaque bacteria can harm teeth, while many gut bacteria help you.

Wait, then leave

Leftovers move through the large intestine slowly, often taking a day or more. Peristalsis keeps them moving. Feces collect in the rectum, the last part of the large intestine.

When the rectum fills, you feel the need to use the bathroom. A ring of muscle around the anus holds the opening closed until you are ready. It relaxes to let feces leave.

Too hard or too runny

If feces stay too long, the large intestine can take back too much water. The feces become hard and difficult to pass. This is constipation.

When an upset gut moves leftovers too quickly, less water may be taken back. The feces can become loose and watery. This is diarrhea. You lose extra water with diarrhea, so drinking extra fluids matters.

Picture two pieces of the same wet sponge. More squeezing leaves one piece drier; less leaves the other wetter. Your intestine takes water through its wall, but this comparison helps you remember how water changes the leftovers.

Urine, or pee, follows a different route. Kidneys make urine by removing waste from blood. Feces leave the digestive tract through the anus.

One bite changes as it moves through your body. A clear story names each place, says its job, and keeps the places in order.

Build a flow chart

Write the pathway with arrows between its parts:

Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus.

Add a job beneath each name: chew and wet; push along; churn; finish digestion and absorb nutrients; take back water; store; let waste out.

Add the helpers beside the right places. Salivary glands send saliva into the mouth. The liver makes bile, the gallbladder stores it, and the pancreas adds juice to the small intestine. Food does not take a side trip through these helpers.

Draw your own first try, then check it against a labeled body picture. If your drawing sends food from the stomach straight to the large intestine, add the missing small intestine and repair the arrows.

Give the journey time

Imagine eating lunch at noon. It might remain in the stomach until about two to four in the afternoon. It then spends several more hours in the small intestine. Leftovers may spend a day or more in the large intestine.

The whole trip usually takes about one day to several days. It varies with the person and the food. Your blood can already be carrying nutrients from a meal while other leftovers are still inside the tract.

Make a simple model

Break crackers or bread into pieces to stand for chewing. Put them into a zip-top bag with a little water. Squeeze the closed bag to show stomach churning.

Over a tray, move the mush into one leg of a clean old pair of tights. Squeeze it along the tube to model peristalsis. Liquid passing through the fabric stands for useful substances leaving the tract. The material left inside stands for feces. Treat everything used in the model as an experiment: do not eat it, and wash your hands afterward.

The model leaves out real digestive juices and helpful bacteria. Its tube is much shorter than your tract. Liquid drips into a tray, but real nutrients pass into the body and are carried away.

Muscles move food, blood transports nutrients, and your brain helps you notice hunger, fullness, and the need for the bathroom.

What you eat, drink, and do helps your digestive system work. Small daily habits can help your teeth at the same time.

Give your gut fiber and water

Fiber is the part of plant food your body cannot digest. It helps keep feces soft and moving. It also feeds helpful gut bacteria.

You get fiber from fruits, vegetables, beans, nuts, and whole grains. Oatmeal and whole-wheat bread are examples. A meal can include useful fiber even though your body does not digest that part itself.

Water also helps keep feces soft. Drinking enough makes it easier to pass them. Water between meals is kind to your teeth too, because it adds no sugar.

Move, chew, and listen

Be active every day. Playing tag, riding a bike, or running helps the muscles of your intestines keep food moving.

Eat slowly and chew well. Small, soft pieces are easier to swallow and for digestive juices to work on. Eating slowly gives your brain time to notice that you are full.

Use the bathroom when your body tells you to. Holding feces in for a long time lets more water leave them. Harder feces are more difficult to pass.

Keep germs off food

Wash your hands with soap and water before eating and after using the bathroom. Germs from hands can get onto food and into your mouth. Stomach acid kills many germs, but some can survive and upset your gut.

Choose a snack for both jobs

Compare an apple and water with chips and soda. The apple provides fiber for your gut. Water helps keep feces soft. The soda adds sugar and acid that can harm teeth. The apple-and-water choice helps both systems.

You can also pair whole-grain toast with water or have plain yogurt and fruit. Think about fiber for the gut, water to drink, and fewer sugary snack times for teeth.

Tell a trusted adult if strong belly pain does not go away, you see blood in your poop, or you keep throwing up. They can help you get care.