Airlock - This is a small plastic tube bent in an S-shape with two bubbles in it, one on either side of the bend. Fit the grommet into the hole provided in the lid of the fermenter, insert the airlock into the hole and attach the lid to the fermenter. Then pour clean water into the airlock until it comes up to about the halfway mark on the two bubbles. The airlock now acts as a valve which allows gas to escape from the fermenter but will not allow air from the outside to get back in. The purpose is to prevent wild yeasts and bacteria getting into your brew to compete with the brewing yeast. Also the airlock lets you know if fermentation is working