Inside a tooth and how decay happens
The four kinds of teeth, the parts of a tooth, plaque and acid, and how enamel repairs itself. With a fair test of drinks on eggshells.
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Adults can have 32 permanent teeth: 8 incisors for cutting, 4 canines for tearing, 8 premolars for mashing and 12 molars for crushing. The last molars at the back are the wisdom teeth.[5]
The part of a tooth you can see is the crown. It is covered in enamel, the hardest substance in the body. Under the enamel is dentin, which forms most of the tooth.[5][4]
In the middle is the pulp, with nerves and blood vessels. The root sits in the jawbone, covered by a bone-like layer called cementum.[5][4]
Plaque is a sticky, colourless film of bacteria. Each time we eat or drink sugar or starch, the bacteria make acid, and the acid attacks the enamel.[3][2]
Enamel loses minerals during an acid attack and gains them back afterwards, all day long. Calcium and phosphate from saliva and fluoride help it repair. A white spot can be a sign of early decay, which can still be stopped.[2][3]
If the attacks keep winning, the enamel breaks down and a cavity forms: permanent damage a dentist has to fill. Having fewer snacks between meals means fewer acid attacks, and fluoride makes the outer enamel tougher.[2][3][6]
Try it: Which drink attacks an eggshell?
You need
- 5 hard-boiled eggs (cooled)
- 5 clear cups, labelled
- The same volume (for example 200 mL) of: water, milk, cola, orange juice, white vinegar
- Measuring jug
- Kitchen roll
- A magnifying glass
Safety
Use hard-boiled, cooled eggs; an adult does the boiling. Do not eat the eggs or drink from the cups afterwards. Vinegar stings in eyes: wash hands after handling. Check for egg and milk allergies. Pour the drinks away down a sink at the end.
What to do
- Write your question: which drink changes the eggshell most? Predict the order from most to least.
- Plan a fair test: what will you change (the drink), what will you measure or observe (bubbles, shell feel and colour), and what will you keep the same (size of egg, volume of drink, time, temperature)?
- Put one egg in each cup and cover it with the same volume of drink. Keep one egg in water as the control.
- Watch for 5 minutes. Which cups show bubbles on the shell?
- After 24 hours lift each egg out, pat it dry and compare: is the shell stained, rough, thinner or soft? Record your results in the table and compare them with your prediction.
What happens, and why?
The eggshell is made of calcium carbonate and is a rough model of enamel. Acids in vinegar and in fizzy drinks react with the calcium carbonate, making bubbles of carbon dioxide and thinning the shell. Water is the control: if the water egg does not change, the changes in the other cups are caused by the drinks. Real enamel is a different mineral and saliva helps repair it, so the egg shows the idea of an acid attack, not exactly what happens in your mouth.[10][11][2]
Play: Name the part
Each card describes a part of a tooth. Is it the enamel, the dentin, the pulp or the root?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16