A physical process which takes a nucleus of a heavy element (like Uranium or Plutonium, for example) and breaks them down into two or more smaller nuclei. This process releases large amounts of energy. It is sometimes called "Splitting the Atom". This is used in many modern power plants to generate electricity by heating water with the energy released. The two or more smaller nuclei which are produced are frequently toxic, and nearly always radioactive, which makes this method of producing electricity controversial. Fission is also the method used in simple nuclear weapons.