At CERN in 1974, scientists created the first version of GEANT (Geometry and Tracking) to help physicists create simulations. Today it is in its fourth iteration, developed by an international collaboration of about 100 scientists from 19 countries. Anyone can download the system to a personal computer, use C++ programming language to plug in details about the particle and material in question and find out what will happen when the two meet. (Illustration by Sandbox Studio, Chicago with Shawna X.)