Geometry - interactive proofs with animations

http://www.gogeometry.com/menelaus1.htm

The Menelaus Theorem

http://www.cut-the-knot.org/Generalization/Menelaus.shtml#proof

Einstein wrote, "Although the first proof is somewhat simpler, it is not satisfying. For it uses an auxiliary line which has nothing to do with the content of the proposition to be proved, and the proof favors, for no reason, the vertex A, although the proposition is symmetrical in relation to A, B, and C. The second proof, however, is symmetrical, and can be read off directly from the figure."

http://www.cut-the-knot.org/Generalization/MenelausByEinstein.shtml