As Janie's original post has created so much interest over the years, I thought I'd add some beginner's thoughts.

It seems to me the clever thing about this method is the clay pot releasing just the right amount of water to keep the medium damp but not wet. I'm using perlite, based on the idea that it can only hold x amount of water within its structure, so that in theory it's impossible to overwater. However, because it floats, you're never sure how much water may be swilling about at the base of a sealed container.

I've soaked and then strained the perlite in a sieve and put it in the container with the clay pot, but I guess beginners always worry that it won't be damp enough at the surface. My observation is that water wicks upwards through perlite to a height of at least 4", but I'd be interested to know the thoughts of others on the subject.

Also, belated thanks to Janie for the original post