(September 30, 2013) Leonard Susskind presents an example of rotational symmetry and derives the angular momentum operator as the generator of this symmetry. He then discusses symmetry groups and Lie algebras, and shows how these concepts require that magnetic quantum numbers - i.e. spin - must have whole- or half-integer values. Originally presented by the Stanford Continuing Studies Program. Stanford University: www.stanford.edu/ Continuing Studies Program: csp.stanford.edu/ Stanford University Channel on YouTube: www.youtube.com/stanford