system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do cut apples, pears and other fruits become brown and mealy on the surface of the cut when left untouched for a while?
gpt
Its because the flesh of the fruit oxidizes.  Oxygen from the air interacts with the fruit.  Its not unlike how metal will oxidize (rust).

Oxygen is actually a fairly dangerous and destructive element.  The skin of the fruit protects the meat from oxygen (among other things).