Drinking Brid
Q. There are many many explanation of the drinking bird on the web - just google "Drinking Bird" and you'll find more information than you could ever want! Here I'll write only a brief description of what causes the bird to drink.
A. First, the bird, with his wet beak, sways in the air. As the water evaporates it cools the air in the bird's neck. Cooler air has less thermal energy and so the air molecules move around less. As we saw when we dipped a balloon in liquid nitrogen, cooled air exerts less pressure (the cool molecules move slower and are no longer able to push out on the balloon and keep it inflated). So, the air in the neck exerts less pressure on the alcohol (the red liquid inside the bird), allowing the alcohol to rise. The rising alcohol moves the center of mass of the bird up. Eventually the center of mass rises above the pivot point and the bird is no longer statically stable. The swaying of the bird moves the center of mass away from the pivot (the base of support of the bird) and the bird falls over. This allows the bird to tip back into the water, which allows the process to continue, and also "resets" the bird, equalizing the pressure and returning the liquid to the bulb at the bottom.
But isn't this perpetual motion?
No! Perpetual motion would mean that this could go on forever without ever adding anything to the system. It may not look like we're adding anything to the system, but in fact by having the bird in an open room we are constantly supplying dry air to evaporate the water from the bird's beak. If we enclosed the bird it would eventually stop when the air couldn't dry the bird's beak anymore.