system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the flesh of an apple go brown fairly quick out in the open?
gpt
It's exposed to oxygen which activates enzymes (Things that break down parts of other things saliva for example contains a lot of enzymes and  it breaks down food in your mouth) that turn it brown in the process. It mainly happens when the skin is not protecting the flesh as oxygen starts the reaction.