Airlocks (or fermentation traps) are U-shaped devices that act as a simple valve allowing carbon dioxide to escape from wine as it ferments while simultaneously keeping oxygen and air-born bacteria from reaching the wine. Typically, sodium metabisulfite solution (instead of water) or alcohol is placed in the airlock "U". This allows gas to escape from the wine. Fruit flies can carry harmful bacteria that may contaminate the wine. When a fly falls into an airlock full of metabisulfite solution, the "meta" will sterilize the bacteria and assuring that the wine will not turn into vinegar.