All matter consists of atoms. Atoms consist of a nucleus containing neutrons and protons (positive charge), and electrons (negative charge) circling around the nucleus. Radio-active elements have to many particles or to much energy in the core, they are unstable. They solve this problem by emitting particles or energy. This is radiation. Some elements are naturally radio-active, for example Kalium, Uranium, Thorium and Radium. Other elements can be made radio-active inside a nuclear reactor, like the one at the RID in Delft.