Bananas turn brown when left outside after a while because certain chemicals in the fruit react with the oxygen in the air by destroying cells, creating that icky brown color. This interaction eventually forms a chemical that, weirdly enough, is similar in structure to the pigment found in sun-tanned human skin.
But as disturbing as that sounds, it's all perfectly natural. After all, oxygen is a very reactive element and combines with many substances in a reaction called oxidation. Bananas, apples, peaches and pears are fruits that are easily oxidized.
Before you dispose of any discolored fruits that look, well, disgusting, keep in mind that they're still edible in most cases as long as they're not too brown and squishy. If you still frown on browning, though, try squeezing a bit of lemon juice on your fruit. The vitamin C (ascorbic acid) in the lemon acts as an antioxidant--a food additive that slows down the reaction between the substances in the fruit and the oxygen in the air. As a result, the lemon juice helps prevent browning and preserves the taste and color of the fruit. Bakers give this sour substance their vote of confidence by using it in recipes for apple pie; this ensures that the apples look every bit as good as they taste.