system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does food in the fridge stay good for a few days and then one day just go bad.

Some food seem to be fine one day and the next just smell or look weird the next.
gpt
Bacteria multiply by binary fission, which means they double when they reproduce and therefore exhibit exponential growth. When you first put the food in the fridge there are only a few bacteria, and when they double there are only a few more. By the time there are a few hundred thousand, this exponential growth means it wont take long for there are millions then billions spoiling your food. 
E.coli, for example, has a doubling time of 20 mins under ideal conditions. Refrigeration will slow most bacterial growth increasing the doubling time.