I presented yesterday at the APS april meeting and it was lots of fun! The room was fairly packed and there were a number of other interesting talks in the session that was on the history of physics. One of the most interesting presentations discussed how Tesla viewed theories of the luminiferous ether. During my talk, I discussed how the special relativistic addition of velocities, in one dimension at least, is very simple if you first express the velocities in terms of rapidities. As it turns out, the addition of velocities in two dimensions is much more difficult until you understand what's going on. It seems mysterious at first, but it's not. Simply put, the problem is that if you add a velocity in the east direction with a velocity in the north direction, you'll get a different answer than you will if you add a velocity in the north direction followed by one in the east direction. That's right, change the order that you add velocities in and you get ...