Postprint version. Published in The Physics Teacher, Volume 49, Issue 3, March 1, 2011, pages 180-183.
Copyright © 2011 American Association of Physics Teachers. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Association of Physics Teachers. The following article appeared in The Physics Teacher and may be found at http://dx.doi.org/10.1119/1.3555508.