Chapter 4 of 8

All 8 chapters
  1. 1The skeleton, readunderstood
  2. 2Joints, readunderstood
  3. 3Muscles move bones, readunderstood
  4. 4Teeth, readunderstood
  5. 5Food groups, readunderstood
  6. 6A balanced diet, readunderstood
  7. 7Exercise and rest, readunderstood
  8. 8Keeping food safe, readunderstood

S2.3.4 Chapter 4

Teeth

Why are my teeth not all the same shape?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Name incisors, canines and molars and the job of each
  • Explain how sugar and poor cleaning cause tooth decay
  • Show how to clean teeth with a brush or a chewing stick

Run your tongue along your teeth. The front ones are thin and flat. Next to them are pointed ones. The back ones are wide and bumpy.

Your teeth are not all the same. Each kind has its own job.

Incisors are at the front. They have flat edges. They cut and bite.

Canines are the pointed teeth beside the incisors. They grip and tear food.

Molars are the wide teeth at the back. They are bumpy. They grind food.

Draw it first

Draw your teeth and their jobs

  1. A row of teeth along one side of the jaw, drawn in three steps. The front of the mouth is on the left. First the teeth, with the words front and back. Then the names: two flat incisors, one pointed canine and two wide molars. Then the jobs written on the gum under them: cut, tear and grind. Labels: front, back, incisors, canine, molars, cut, tear, grind.frontback

    1 Draw a row of teeth on a line of gum. Write front and back.

  2. A row of teeth along one side of the jaw, drawn in three steps. The front of the mouth is on the left. First the teeth, with the words front and back. Then the names: two flat incisors, one pointed canine and two wide molars. Then the jobs written on the gum under them: cut, tear and grind. Labels: front, back, incisors, canine, molars, cut, tear, grind.incisorsmolarscaninefrontback

    2 Draw flat incisors, a pointed canine and wide molars. Write their names.

  3. A row of teeth along one side of the jaw, drawn in three steps. The front of the mouth is on the left. First the teeth, with the words front and back. Then the names: two flat incisors, one pointed canine and two wide molars. Then the jobs written on the gum under them: cut, tear and grind. Labels: front, back, incisors, canine, molars, cut, tear, grind.incisorsmolarscaninefrontbackcutteargrind

    3 Under each kind, write its job: cut, tear, grind.

A child has 20 milk teeth. They fall out, and adult teeth grow in. An adult has up to 32 teeth. Some adults have fewer.

A bar chart of teeth. The bar for milk teeth reaches 20. The bar for adult teeth reaches 32. The scale counts teeth from 0 to 40 in tens. Labels: teeth, milk, adult.010203040milkadultteeth
A child has 20 milk teeth. An adult has up to 32 teeth.

Look inside a tooth. The outside is very hard. The inside is softer. The root holds the tooth in the gum.

A tooth cut in half, standing in the gum. The outside of the tooth is a thin hard shell. The inside is hatched. Two roots go down into the dotted gum. Labels: hard outside, inside, root, gum.hard outsideinsiderootgum

Tiny germs live on your teeth, in a sticky layer. Germs are living things too small to see. They feed on sugar. They make acid.

The acid makes holes in the hard outside of a tooth. These holes are tooth decay.

The same tooth with decay. A black hole has eaten into the hard outside at the top. Labels: hard outside, inside, root, gum, hole.hard outsideinsiderootgumhole

See it three ways

As a picture

Four boxes one under the other, with a downward arrow between each pair. Sugar leads to germs, germs lead to acid, and acid leads to a hole. Labels: sugar, germs, acid, hole.sugargermsacidhole

In words

Sugar feeds the germs. The germs make acid. The acid makes a hole in the tooth.

In symbols

A child has 20 milk teeth. An adult has up to 32 teeth.

Cleaning takes the sticky layer away. Brush your teeth twice a day. Brush for two minutes. Brush every side of every tooth.

Your teeth in four parts. The top teeth have part 1 and part 2. The bottom teeth have part 3 and part 4. Each part is brushed for 30 seconds. Labels: top teeth, bottom teeth, 1, 2, 3, 4, 30 seconds each part.1234top teethbottom teeth30 seconds each part
Four parts, 30 seconds each, make two minutes.

No brush? Use a chewing stick. Chew one end until it is soft, like a brush. Then rub your teeth with it. Use a fresh end each day.

Rinse your mouth with clean water after sweet food. Eat sweet food less often.

Try it

What does acid do to a shell?

You need: a clean eggshell from a cooked egg, two clean cups, water, vinegar, a spoon.

Be safe: Never taste or drink anything in this activity. Throw the shell and the liquid away. Wash your hands.

  1. Break the shell into two bits. Wash your hands afterwards.
  2. Put one bit in a cup of water.
  3. Put the other bit in a cup of vinegar.
  4. Leave both cups for a day or two.
  5. Lift each bit out with the spoon. Press each one gently.

What you will see: The bit in water is still hard. The bit in vinegar is soft and thin. Vinegar is an acid. Eggshell is hard like a tooth. Acid wears it away. Acid from germs wears away teeth in the same way.

If you cannot try it, this is what happens.
Two cups. In the left cup, labelled water, a thick bit of shell lies in water. Under it is the label shell stays hard. In the right cup, labelled vinegar, a very thin bit of shell lies in vinegar. Under it is the label soft and thin. Labels: water, shell stays hard, vinegar, soft and thin.watershell stays hardvinegarsoft and thin
A row of milk teeth on the gum with the middle one gone. The teeth beside the gap stand straight. A dotted box marks the room, with the label room for the next tooth. Labels: room for the next tooth.room for the next tooth
The same row after the middle tooth was lost too soon. The two teeth beside the gap lean into it, and there is no room. Labels: teeth lean in, no room.teeth lean inno room
A milk tooth lost too soon leaves less room.

A wrong idea many people have

Milk teeth do not matter, because they fall out.

Why it feels right: Milk teeth are only used for a while. New teeth come later.

The test that breaks it: Stand some books in a row, close together. Take one book out and leave a gap. The books beside it lean into the gap. Teeth do the same. If a milk tooth goes too soon, the next teeth lean in. The adult tooth has no room.

What is true: Milk teeth keep the space for adult teeth. A bad milk tooth also hurts and can make you ill. Clean them too.

Key words

incisors
flat teeth at the front. They cut and bite.
canines
pointed teeth. They grip and tear.
molars
wide teeth at the back. They grind food.
tooth decay
holes in a tooth, made by acid from germs.

Back to your guess

The answer is C: Different teeth do different jobs.

Different teeth do different jobs. Some cut, some tear and some grind.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which teeth are pointed and tear food?

    Tap your answer.

    1. Not this one. Incisors are flat at the front. They cut and bite. Try another.

    2. Yes, that is right. Canines are pointed. They grip and tear food.

    3. Not this one. Molars are wide and bumpy at the back. They grind. Try another.

    See the answer

    Canines

    Canines are pointed. They grip and tear food.

    • If you chose A: Incisors are flat at the front. They cut and bite.
    • If you chose C: Molars are wide and bumpy at the back. They grind.

  2. Joseph chews a hard bit of cooked cassava. Which teeth grind it?

    Tap your answer.

    1. Not this one. Incisors cut and bite. The wide back teeth do the grinding. Try another.

    2. Not this one. Canines grip and tear. They are not wide enough to grind. Try another.

    3. Yes, that is right. Molars are wide and bumpy. They grind food.

    See the answer

    The molars at the back

    Molars are wide and bumpy. They grind food.

    • If you chose A: Incisors cut and bite. The wide back teeth do the grinding.
    • If you chose B: Canines grip and tear. They are not wide enough to grind.

  3. What do germs on your teeth make from sugar?

    Tap your answer.

    1. Yes, that is right. Germs feed on sugar and make acid. The acid makes holes in the tooth.

    2. Not this one. Germs make acid. Water does not make holes in teeth. Try another.

    3. Not this one. Germs do not make teeth. They make acid, and acid makes holes. Try another.

    See the answer

    Acid

    Germs feed on sugar and make acid. The acid makes holes in the tooth.

    • If you chose B: Germs make acid. Water does not make holes in teeth.
    • If you chose C: Germs do not make teeth. They make acid, and acid makes holes.

  4. Mei brushes four parts of her mouth. She brushes each part for 30 seconds. How long does she brush in all?

    See the answer

    120 seconds, which is 2 minutes

    Four times 30 is 120 seconds. There are 60 seconds in a minute, so this is 2 minutes.

  5. From an earlier chapter

    Which muscle pulls to straighten your arm?

    Tap your answer.

    1. Not this one. The front muscle bends the arm. It goes soft when the arm straightens. Go to S2.3.3. Try another.

    2. Yes, that is right. The triceps pulls to straighten the arm. The biceps pulls to bend it.

    3. Not this one. A tendon is a cord that joins a muscle to a bone. It does not pull by itself. Go to S2.3.3. Try another.

    See the answer

    The muscle at the back of the upper arm

    The triceps pulls to straighten the arm. The biceps pulls to bend it.

    • If you chose A: The front muscle bends the arm. It goes soft when the arm straightens. Go to S2.3.3.
    • If you chose C: A tendon is a cord that joins a muscle to a bone. It does not pull by itself. Go to S2.3.3.

  6. Show how to clean your teeth with a chewing stick. Write the steps.

    See the answer

    Chew one end until it is soft. Rub all sides of every tooth. Use a fresh end each day.

    The soft end works like a brush. It takes away the sticky layer where germs live.

  7. Hana has no toothbrush. What can she use to clean her teeth?

    Tap your answer.

    1. Yes, that is right. The soft end works like a brush. Rub every side of every tooth. Use a fresh end each day.

    2. Not this one. A soft chewing stick works like a brush. It takes the sticky layer away. Try another.

    3. Not this one. A sharp stick can hurt your gums. Chew one end soft, then rub your teeth with it. Try another.

    See the answer

    A chewing stick, chewed soft at one end

    The soft end works like a brush. Rub every side of every tooth. Use a fresh end each day.

    • If you chose B: A soft chewing stick works like a brush. It takes the sticky layer away.
    • If you chose C: A sharp stick can hurt your gums. Chew one end soft, then rub your teeth with it.

  8. Ravi says, "I need not clean milk teeth." What is true?

    Tap your answer.

    1. Not this one. Milk teeth keep the space for adult teeth. A bad milk tooth also hurts. Try another.

    2. Not this one. Germs live on every tooth. Milk teeth can decay too. Try another.

    3. Not this one. Germs live on every tooth, including the back ones. Try another.

    4. Yes, that is right. Milk teeth can decay and hurt. A milk tooth lost too soon leaves the next tooth less room.

    See the answer

    Clean them too. They keep the space for adult teeth.

    Milk teeth can decay and hurt. A milk tooth lost too soon leaves the next tooth less room.

    • If you chose A: Milk teeth keep the space for adult teeth. A bad milk tooth also hurts.
    • If you chose B: Germs live on every tooth. Milk teeth can decay too.
    • If you chose C: Germs live on every tooth, including the back ones.

Keep it

Copy this into your exercise book.

Incisors cut. Canines tear. Molars grind. Germs on teeth feed on sugar and make acid. Acid makes holes. Brush twice a day for two minutes, or use a chewing stick.

Draw: A row of teeth. Label the incisors, canine and molars.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (704 KB). It opens on any phone.