How to Tell a Neutron from a Gamma Ray in a Water Tank
In a tank of pure water, particles traveling faster than light in that medium leave bluish flashes, like a plane breaking the sound barrier creates a sonic boom. From these flashes, you can tell what kind of visitor passed through: a gamma quantum or a neutron. This is no idle task: at borders and warehouses, you need to quickly distinguish a harmless radioactive cargo from weapon-usable material.