Thank you! Do you think vinegar would work, too?

hla nailed it, SproutMom. It's called enzymatic browning. Apples, peaches, avocados, and even beets, contain an enzyme which when ruptured by slicing and thus exposed to air, turns brown. Slice them and toss them into a bowl of water into which you've squeezed a whole lemon. And then toss the lemon rind in for good measure.

Its oxidation, baby! Just like apples or potatoes. Same thing happens to red beets, but the red color overpowers the brown. I'd assume a little acid would prevent the reaction.