If your avocados aren't ripe enough, you can expedite the process by using a piece of fruit and a paper bag. Put the avocado in the bag with a banana or an apple, then seal the bag for several hours or overnight. The ethylene gas that these fruits produce is a hormone that aids the ripening process. For an even faster fix, you can also ripen avocados in the microwave. Halve and then pit the avocado. Wrap each half in microwave-safe plastic wrap and then microwave on high for 2 minutes.