No matter how crisp and juicy an apple may be, no one will want to eat it once it's turned brown. Apples brown when the flesh of the fruit is exposed to oxygen, and it's enough of a problem that that the USDA just approved an apple that's genetically modified to not brown when sliced (it's America's first approved genetically modified apple—yay?). That apple won't be in markets until at least 2016. Until then, here are a few old-fashioned tricks to prevent sliced apples from oxidizing:
CITRUS OR PINEAPPLE JUICE
What do these juices have in common? They contain citric acid, which prevents oxidation and browning. If you're going the citrus route, simply squeeze your lemon, lime, or orange directly onto the surface of the cut apple. Otherwise, soak the fruit in the juice. Whichever juice you use, be aware that it will impart a little bit of flavor onto the apple, so pick your favorite.
SALT & WATER
Dissolve 1/8 teaspoon of salt into one cup of water, add your apple slices, let them soak for a few minutes, then drain them. Give the slices a quick rinse in fresh water after you drain them, so your fruit won't taste salty.
HONEY & WATER
Stir two tablespoons of honey into one cup of water and soak your apple slices in the mixture for 30 seconds. This works because there is a compound in honey that stops the enzyme responsible for oxidization.
See some great ways to eat and cook with apples:
Best things to make with apples
The easiest ways to keep your apples from browning