On containers filled with water, these airlocks create a liquid seal that keeps contaminants out while letting gases produced by chemical reactions and temperature changes escape. They’re common in fermentation applications, such as beer brewing and winemaking. Use them with venting flasks, carboys, and other containers that are sealed with a rubber stopper; to install, push the stem into the rubber stopper’s hole. At the top, these airlocks have a cap that blocks dust, but still lets gas out. When it’s time to clean them, they come apart in three pieces, so you don’t need a specially-shaped brush to reach every crevice.