A water molecule is made up of two hydrogen atoms and one oxygen atoms to form H₂O, but not all hydrogen is the same. Most hydrogen atoms consist of one proton and one electron, but a very few of them also contain a neutron. This makes the hydrogen atom twice as heavy as a regular hydrogen atom. If both of the hydrogen atoms in a water molecule contain that extra neutron, then it is called "heavy water". Because those hydrogens only make up a small amount of the actual mass, heavy water is not much heavier than regular water, but it does have some interesting properties. One of those properties is the ability to slow down neutrons, which improves the fission of Uranium-235. This was very important in the development of the atomic bomb, and was also important in the development of nuclear power plants.