Sometimes your produce merchant's entire supply of fruits such as apples, avocados, and peaches is unripe. This is not intolerable problem as long as the fruit is not more than slightly unripe and you give it time to ripen further. The additional ripening will make the fruit taste sweeter because some of the starches will be chemically converted into sugar. In addition, the fruit will likely lose acidity, making it taste comparatively sweeter. But if you let these fruits finish their ripening process at home, you run the risk of having them spoil before they become fully ripe. Your best solution is to hasten the ripening process by harnessing the ethylene gas that the fruit naturally generates. Rather than letting the gas dissipate into your kitchen air, trap it by placing the fruit in a paper bag. The ethylene gas becomes concentrated and therefore accelerates ripening.
Be sure to pierce the bag with a half dozen or so well-distributed pencil-sized holes, for in order to stay sound, your fruits need to "breathe". Like animals, they take in oxygen and expel carbon dioxide (though the non-fruit part of the plants does the exact opposite.)