Teeth are the hardest thing in the human body. 牙齿是人体中最坚固的东西。

　　They are not made of bone. 它们不是由骨骼构成的。

　　They are made of enamel, gum and so on. 而是由牙釉质，牙龈等构成的。

　　People use teeth when they eat something or tear something. 人们用牙齿吃东西或撕东西。

　　How many teeth do people have?  人有多少颗牙呢？

　　When they are young, they have 20 teeth. 小时候，人有20颗牙。

　　And when they get older, they have 32 teeth all together. 长大后，人总共有32颗牙。