While most firm kiwis are fine to eat, the fruit becomes sweeter as it softens and ripens. If you wish to soften a rock-hard kiwi quickly, the fastest method is to expose it to ethylene gas, a chemical in fruits and vegetables. Some fruits contain higher amounts of this chemical. Once exposed to the gas, a kiwi can quickly ripen within a few days.
Place the kiwi in a brown paper bag.
Add an apple, banana or pear to the brown paper bag. Place the other fruit right beside the kiwi.
Fold over the top of the bag and place it on your countertop.
Open the bag and check it daily to see if the kiwi is ripe. When ripe, the kiwi should yield to light pressure.
Remove the kiwi from the bag and eat, or place in the refrigerator to keep it from becoming overripe.