Heavy water actually has multiple, rather complicated, yet amazing uses. It is used as a neutron moderator, which means heavy water is put in reactors with neutrons and uranium. The heavy water slows down the neutrons and they react with said uranium. Light water can also be used as a neutron moderator, but not as efficiently, because it absorbs more neutrons. This is because it is lacking a neutron in it`s nucleus, whereas heavy water already has this neutron, and therefore will absorb less. Metabolic rate testing is another very common use of heavy water. Biologists and/or physicists replace both the hydrogen and oxygen parts of water with an uncommon isotope of each of these elements. This makes the non-reactive, more easily traceable deuterium oxide-18 (D218O). Since these deuterium oxide is so uncommon, it is easier to trace than regular water once ingested. This water is essentially `tagged` because there won`t be any other water like it in the person`s body. Scientists can then use this to calculate the individual`s metabolism based off measurements of their oxygen intake and carbon expulsion. Finally, a very rare and expensive use of heavy water is tritium production. Tritium helps with a number of things, but most commonly self-lighting. Some tritium is produced in heavy water moderated reactors, which use heavy water as a coolant and moderator, much like a neutron moderator.