Canup spends her days tweaking computer code and watching colored particles spin on cue. For her moon simulations, she creates an Earth and a protoplanet, and gives each a specific mass, velocity, temperature and other characteristics. Then she programs a collision and sits back to watch the show. The math is so complex, it takes a 2GHz processor a week to compute each scenario. Recently, by running a simulation backward, Canup found that most of the moon probably came not from the Earth but from a specific area on the protoplanet that hit it. Her next step is to add "particle splitting": Partway through a simulation, she'll program it to ignore humdrum information (say, the particles at Earth's core) and focus on the interesting stuff (particles that might clump together as the moon).