Why does one ripe banana make the fruit around it ripen faster?

Because ripe fruit breathes out a gas called ethylene, and ethylene is a ripening signal. It is a plant hormone, the first gas ever found to act as one, and fruits like bananas and tomatoes respond to it by turning sweeter, softer and less green. Even better for the effect, ethylene can trigger fruit to make more ethylene. So a ripe banana in a fruit bowl nudges its neighbours, they start producing the gas too, and the whole bowl speeds up. A covered bowl or a paper bag traps the gas and makes it work faster still.

Not every fruit plays along. Climacteric fruits, such as bananas and tomatoes, keep ripening after they are picked, which is why they can be harvested green and shipped. Non-climacteric fruits only ripen on the plant.

People used ethylene long before anyone knew it existed. Ancient Egyptians gashed figs to ripen them, and wounding a fruit makes it release ethylene. The ancient Chinese burned incense in closed rooms to ripen pears. Florida farmers ripened crops in sheds warmed by kerosene lamps and credited the heat, until Frank Denny showed in 1924 that it was the ethylene from the lamps. Today, bananas cross the oceans green and are gassed with ethylene once they arrive.