When fruit is picked or when the weather signals it to begin ripening, it releases ethylene gas. This chemical causes the fruit’s cells to make all the changes involved with ripening: Chlorophyll is broken down, causing color changes; starches are converted into sugars, making the fruit sweet and less mealy; acids are broken down, causing it to taste less sour; and pectin is broken down, making it softer.
Putting the fruit in a loosely closed paper bag traps the ethylene gas inside and accelerates ripening. Don’t seal the bag too tightly or use a plastic bag, however—if you trap too much moisture, mold may grow. If you really want to ripen things in a hurry, put an apple in the bag with your other fruit—they produce a lot of ethylene gas.