Animated particle tracks showing the production and decay of pions in a bubble chamber. A negatively-charged kaon (blue) enters from left and decays to produce a neutral pion (which leaves no track) and a negative pion (yellow). The neutral pion immediately decays into two gamma ray photons which also have no charge and leave no track. Their paths can inferred when they strike a lead sheet (grey), producing an electron (red) and its antiparticle, a positron (green).