The dynamics of colliding galaxies are reasonably well understood. Current models are producing more and more configurations that match the more exotic galaxy shapes observed by modern surveys, so we seem to have it down. This is old news, but what amazes me is that while these simulations used to require massive processing power on supercomputers, you can now run your own simulations with a java app in your web browser. Click through to http://burro.cwru.edu/JavaLab/GalCrashWeb/backgrnd.html, at the Astronomy Department of Case Western Reserve University, and start playing! The simulator presents you with two spiral galaxies and lets you choose their relative sizes, positions and angles. Click ‘Start’, and watch the simulation unfold live before you. Great fun, especially if you manage to produce a copy of one of the real life examples shown to the left!