I recently read an article in Gourmet magazine that advised against using "tomato-shaped rocks" that are found in your corner grocery store. Well, what do you do if you don't have a farmer's market close by, or it's the dead of winter and you're- GASP- eating a tomato out of season? Or your bananas are neon-green and chalky but you don't want to wait a week for them to ripen? How can we make the most with what we've got?
Enclose the fruit in a paper bag with another already ripe fruit (it need not be the same fruit). The ripe fruit emits ethylene gas and your target fruit will soften and sweeten because of its concentrated environment inside the paper bag. Tomatoes and bananas are climacteric fruit, meaning they can be harvested mature but hard, but then exposed to ethylene gas to ripen then. This after-harvest ripening doesn't always happen, but this little paper bag trick can improve dramatically the quality of the fruits we buy. Pears, avocados, and kiwis are the other climacteric fruits that will ripen well with the paper bag enclosure.