system
Given the following post, respond as if you're commenting on Reddit:
human
Why do fruits get bruised?

Specifically Apples, Pears, and Bananas. If they get bumped or banged around, why do they discolor on the inside?
gpt
Fruit contains an enzyme called polyphenoloxidase as well as phenolic compounds.  When the fruit is intact they are seperated by cellular structure of the fruit.  When the fruit is damaged they come in contact and the polypholoxidase oxidizes the phenolic compounds.  The newly oxidized phenolic compounds turn into brown spots.

Another way for the phenolic compounds to oxidize is if they are exposed to the oxygen in air by breaking the skin.  This is why a half eaten apple quickly turns brown on the exposed interior flesh surface.

Edit:
You'll also notice that it is much more difficult for oranges to become bruised.  This is because citric acid can be oxidized very easily and will "steal" the oxygen from oxidizing the phenolic compounds.  If you dip apple slices in lemon juice it will take them much longer to turn brown for this reason.