Compare the fruit. Taste one piece from the bag and one from outside the bag. The fruit in the paper bag should be quite a bit riper than the fruit that you left out. Wait another day and taste the last two pieces. By this time, the fruit in the bag may be over ripe, while the other piece will be about the same as when you started.
Plants produce a gas called ethylene.
This gas causes the plant to produce chemicals called enzymes, which do several things. They cause the starch and acids in the fruit to change into sugar. This makes the fruit more sweet and less sour. They also weaken the cell walls, causing the fruit to soften. Apples produce a lot of ethylene gas as they ripen.