To make avocados ripen quickly, place them in a brown paper grocery bag and close the top by rolling it over several times. Leave the bag at room temperature for several days, and then check the contents. The darker the skins, the riper the avocados.
Avocados release ethylene gas as they ripen, and placing them in a paper bag traps the gas around the fruit, causing it to ripen more quickly. To further hasten the ripening of an avocado, add an apple, tomato or banana to the brown paper bag. These fruits release additional ethylene gas, which helps the avocado ripen.