Superior vintage style airlock with cap.

 Airlocks are designed to allow the gas during fermentation to escape, without letting air back into the fermenter

They should always have water in them to create the seal.

The airlock is fitted into a rubber or silicone bung or a grommit in the fermenting vessel

Fermenting under an airlock is a safe clean way to ferment your wine or beer, ensuring insects and small vinegar flies are kept away.