Usually you race to eat a banana before it over-ripens but on the rare occasion of buying green bananas, you need a ripened one right now. Rather than wait a couple of days for nature to do its business, you can speed things up with the help of a paper bag. Fruits ripen when starches turn into sugars and that happens thanks to ethylene, a gas released in the process. By placing a banana in a paper bag, this gas is semi-trapped and therefore speeds up the ripening process. But there is even a faster method is to use your oven. Warm unpeeled bananas on a baking sheet in an oven at 250°F for 15-20 minutes. It’s not quite as effective as letting bananas ripen naturally, but it will soften and sweeten your fruit in a pinch.