Step 6: Air Lock
I used a commercial air lock but you could fashion an air lock from a piece of clear vinyl tubing, one end fitted to the cap and the other end in a cup of water. The commercial air locks are only $1 or so and are included in the starter kits. I drilled a 3/8" hole in the water bottle cap. The air lock fits snugly in the hole. The idea is to let carbon dioxide produced during fermentation escape while keeping air outside from getting in.