How do you joint copper pipe to qua pex pipe?

The best method, approved for behind the wall installation is using something like a "sharkbite" connector. The manufacturer of these can be found at the following website:

http://www.cashacme.com/prod_sharkbite.php

These can be purchased through your local hardware store such as Ace, or are available at Home Depot. I have not seen them at Lowes, but they may be carrying them now.