The rising air bubbles create a vaccum and the vaccum pulls the water through the sponge.
The simplest way to creat one is a small 3/4 x 3 inch tube of pvc. Stuff one end with about 1 inch of filter floss, or use a cotton ball. Drop an airstone in the other end and lay down in the tank on it's side. To ensure the airstone doesn't come back out, silicone the airline tubing to the pvc. To add in biological filtration, add in floss, then some gravel from an established tank and then a little more floss.
Total cost is about $0.50
