The other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are generally referred to as thermonuclear weapons or more colloquially as hydrogen bombs, as they rely on fusion reactions between isotopes of hydrogen. All such weapons derive a significant portion, and sometimes a majority, of their energy from fission. This is because a fission reaction is required as a "trigger" for the fusion reactions, and the fusion reactions can themselves trigger additional fission reactions.