A nuclear weapon (also called an atom bomb, nuke, atomic bomb, nuclear warhead, A-bomb, or nuclear bomb) is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or from a combination of fission and fusion reactions (thermonuclear bomb). A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or from a combination of fission and fusion reactions. Both bomb types release large quantities of energy from relatively small amounts of matter. In early 2019, more than 90% of the world's 13,865 nuclear weapons were owned by Russia and the United States. Plutonium-239 and uranium-235 are the most common isotopes used in nuclear weapons. Each piece by itself was not enough to constitute a critical mass (the minimum amount of nuclear material needed to maintain fission)—but by colliding the pieces, critical mass was reached and a fission chain reaction occurred. Although nuclear weapons have only been used twice in warfare—in the bombings of Hiroshima and Nagasaki in 1945—about 14,500 reportedly remain in our world today and there have been over 2,000 nuclear tests conducted to date.