Cutaway illustrations of the two different types of nuclear weapons used at the end of the Second World War. The bomb dropped on Hiroshima on 6th August 1945 was a gun-type fission weapon code-named 'Little Boy' (left), whereas that dropped on Nagasaki three days later was an implosion-type weapon, code-named 'Fat Man' (right). Little boy achieved fission by shooting a hollow cylinder 'bullet' of uranium-235 into a solid cylinder of the same isotope. To achieve fission of the Fat Man bomb, a number of explosions were initiated around its plutonium-239 core (red sphere).