The Drinking Bird - from "Sixty Symbols"

This video is delightful as a stand-alone discussion of the physics of heat engines, but I'm using it here to introduce a larger reference source. Sixty Symbols is a site set up by the University of Nottingham to store brief videos (like the above), one for each of 60 symbols used in mathematics, physics, or other fields of science. Over 40 of them are already in place. I've just looked at a few, and they are 4+ excellent.

Found through a link at The Centered Librarian, on one of my regular weekly visits.