A tooth is one of the hard, white structures in your mouth that you use to bite and chew food. Humans grow two sets of teeth during their lives: baby teeth (which fall out as you grow) and permanent teeth (which you keep for the rest of your life if you take care of them). Adults have 32 teeth, including molars for grinding, canines for tearing, and incisors for cutting.

Each tooth has a visible crown covered in super-hard enamel (the hardest substance in your body) and roots that anchor it into your jaw. Inside is softer material containing nerves and blood vessels, which is why cavities and injuries hurt so much.

Animals have teeth adapted to their diets. Sharks grow thousands of teeth throughout their lives, replacing them constantly. Herbivores like horses have flat teeth for grinding plants, while carnivores like lions have sharp teeth for tearing meat. Rodents have front teeth that never stop growing, so they gnaw constantly to keep them worn down.

We also use tooth in expressions: something with teeth has real power (like a law with serious penalties), and you might fight tooth and nail when you struggle with everything you have. The jagged edge of a saw has teeth, and so do a comb or a gear.