A 2 liter bottle. I put in a cup of sugar, a package of yeast, then fill it up a little over half way with luke warm water. Make a small hole in the cap and insert a 2-3 foot piece tubbing into the air space in the bottle and seal it off with a bunch of tape. After just a little while the liquid starts to bubble and foam a little, fermenting and making Co2 gas. I place the tube so the gases emited fall directly onto the plant. They seem to like it It last about four days. When the gas coming out starts to smell like alcohol it is no longer making effective Co2, I was told.