<human>:  Why do we have to clean our teeth?
<bot>: Teeth are made of enamel, which is a very hard substance. If you don’t clean your teeth, the enamel will wear away and the dentin underneath will start to show. This is called tooth decay. Tooth decay is caused by bacteria in your mouth. The bacteria produce acids that eat away at the enamel, which is the hard outer layer of your teeth.