Relativity - The Train Paradox (Time Dilation and Length Contraction). This is a famous example of the strangeness of relativity. Demonstrates just how non-intuitive reality can be.
Described here by Professor Richard Muller of the University California, Berkeley. Taken from Lecture 22 of the spring 2006 webcasts of Physics For Future Presidents. Also known as Descriptive Introduction to Physics. Empahsis is on conceptual understanding, rather than mathematics.
Full lecture: http://www.youtube.com/watch?v=nNgzqpKZwhE
