I've always heard that an avocado could ripen faster if put in a paper bag.
These claims can be found in many places on the internet :
Here is a quote from the first link :
Place unripe avocados in a brown paper bag to ripen. This traps the ethylene gas they produce and helps them to ripen. A firm avocado placed in a paper bag will ripen at room temperature in about three to six days. As the fruit ripens, the skin color will darken.
Is that true?