An Airlock (also known as a fermentation lock) is used as part of the fermentation process when brewing your own beer. Commonly, when brewing beer, carbon dioxide will be released. The airlock ensures that it is released from the fermenter as well, while not allowing air to enter the fermenter at the same time, thus avoiding oxidation. This S-Shaped Airlock is designed to fit into the bung, which resides on top of the fermenter. Simply fill it halfway with water or a cheap vodka and let the venting process begin.