Cryogenic fuel is a type of fuel that requires storage at extremely low temperatures to maintain it in a liquid state. Cryogenic fuels are used in machinery that operates in space, such as rockets and satellites, where ordinary fuel cannot be used due to the very low temperatures often encountered in space and the absence of an environment that supports combustion. Cryogenic fuels are most often liquefied gases such as liquid hydrogen. Some rocket engines use regenerative cooling, which is the practice of circulating their cryogenic fuel around the nozzles before the fuel is pumped into the combustion chamber and ignited. Cryogenic engines are the most prestigious rocket engine technology due to their design and operational complexity, and only a few countries have been able to develop this intriguing engine. Liquid oxygen is often mistakenly called cryogenic fuel, but it is actually an oxidizer and not fuel. Cryogenic engines are used in high-performance upper stages and boosters, and the United States, Russia, Japan, India, France, and China are the only countries that have operational cryogenic rocket engines.