Molecular collisions. Computer model showing the paths taken by three molecules before and after two consecutive collisions. Each coloured strand shows the path taken by an atom in a molecule over time. A diatomic (two-atom) molecule collided at upper centre with a poly-atomic (many-atom) molecule entering from top right. The diatomic molecule exited at bottom left, while the poly- atomic molecule collided with another poly- atomic molecule entering from upper left. They then exited at lower right and bottom left respectively. Earlier collisions between molecules are seen in the background. Modelled by Professor Eric Heller, Harvard University, USA.