I don't understand why so many people on here are so against using an airlock as an indication of fermentation. Viewed simply, yeast breakdown sugar into ethanol and carbon dioxide. The C02 waste requires more room, being a gas, than it did when it was carbon and oxygen in the solid sugar molecule. It quickly fills up the area above the beer and creates pressure. If you use a carboy or a bucket with anything close to an airtight seal the C02 will need a means of escape...enter the air lock....

steelerguy