Reverse Osmosis

Rather than explain this myself, here are some links that will help. You may find many others on the web.

This link explains how reverse osmosis works: http://science.howstuffworks.com/question29.htm

This post covers some interesting home experiments on Gore-Tex: http://www.kayakforum.com/cgi-bin/Technique/index.cgi/noframes/read/15222

Many thanks to these contributors.