Scientific American presents Everyday Einstein by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.

A young listener writes in with this question:

“Why do apples turn brown after you cut them?”

An excellent question!

The Secret Life of Fruit
Imagine for a second that you’re an apple. You’re hanging there on a tree, just minding your own business, when you feel it -- the force of gravity has started to overcome the strength of your stem. You break free from the tree and start to fall. As you hurtle towards the earth, you want to scream, but realize you have no mouth. The air rushes past you, and then with a sudden, jarring impact, you are slammed against the ground. You roll for a few inches before coming to a stop, bruised and broken. Without the protection of your peel to protect you, it’s only a matter of time before you start to become overrun with bacteria and fungus.

A lesser fruit would have given up at this point, succumbing to rot. But not you, because your arsenal of defensive weapons isn’t limited to your armor-like peel, you also have access to chemical and biological weapons. One such weapon is a protein called catechol oxidase.

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