system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do teeth rot inside our heads while we're alive, but can remain intact for thousands of years after we die?
gpt
While we're alive, there's bacteria that live on your mouth that feeds on the sugar/starch/foodstuffs that you eat. When the bacteria eat that, they release lactic acid which is one of enamel's greatest weaknesses. Once the bacteria have broken through the enamel, it's only a matter of time before bacteria feast on your nerve. 

Once you're dead, you aren't eating and you don't have live nerve tissue in your teeth to feed the bacteria. They also die, and the teeth survive because enamel is indeed the hardest material in the body.