Avocados do not ripen on the tree, but only after they have been harvested.  You can speed up the ripening process by following this easy step.
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<img src="http://www.lgssales.com/wp-content/uploads/ripe-avocados-300x200.jpg" alt="ripe avocados" width="300" height="200" class="alignright size-medium wp-post-image wp-image-1544" />Place unripe avocados in a brown paper bag with an apple or banana for two to three days until they are ripe. 

This traps the ethylene gas produced and helps them to ripen. As the fruit ripens, its skin turns from green to a dark brown-green color and "gives" slightly when it is gently squeezed.

