Answer : Fruits such as apples, as well as some vegetables including potatoes, turn brown when their flesh is exposed to air for much the same reason iron turns rusty: oxidation. When you peel or cut open an apple or the fruit gets bruised, a family of enzymes called polyphenol oxidases (also called tyrosinases) in the cells is exposed to oxygen. The enzymes then cause other compounds in the fruit to combine with oxygen from the air, turning the exposed surface brown. Although brown fruit is safe to eat, you can prevent this unsightly reaction by minimizing exposure to air (keeping cut pieces submerged in water, for instance) or spritzing with lemon juice or another source of ascorbic acid (vitamin C).
Its not just fruit that tyrosinases turn brown, by the way: The enzymes also regulate the effect of the pigment melanin in human skin, so theyre essential to skin coloration as well as tanning. Defects in the function of these enzymes lead to albinism.