The fermentation lock or airlock is a device used in beer

Beer

Beer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...

 brewing

Brewing

Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...

 and wine making that allows carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 released by the beer to escape the fermenter, while not allowing air to enter the fermenter, thus avoiding oxidation.

There are two main designs for the fermentation lock, or airlock. These designs work when half filled with water. When the pressure of the gas inside the fermentation vessel exceeds the prevailing atmospheric pressure the gas will push its way through the water as individual bubbles into the outside air. A sanitizing solution or vodka

Vodka

Vodka , is a distilled beverage. It is composed primarily of water and ethanol with traces of impurities and flavorings. Vodka is made by the distillation of fermented substances such as grains, potatoes, or sometimes fruits....

is sometimes placed in the fermentation lock to prevent contamination of the beer in case the water is inadvertently drawn into the fermenter.

This device may take the form of a tube connected to the headspace of the fermenting vessel into a tub of sanitized liquid or a simpler device mounted directly on top of the fermentation vessel.

Currently, a popular fermentation lock that mounts on top of the fermentation vessel is the three-piece fermentation lock. Other models contain three bulbous chambers allowing for a broader range of pressure equalization. These bulbous fermentation locks were generally made of hand blown glass and are nowadays often made of clear plastic.