The Fermtech thief is simply a plastic tube with a check valve in the bottom. When placed in liquid, such as beer or wine, the liquid will bypass the check and the thief will rise to the liquid level. Taking the thief out will cause the check valve to seat, allowing liquid to be taken out. A hydrometer can then be placed in the thief, or the liquid can be drained into a test jar by pushing the check up against the side of the jar. This makes taking a gravity reading a snap! It's also great for pulling samples of your beer/wine from your carboy or a barrel.