de Laval nozzle
A de Laval nozzle is a device, designed by Gustav de Laval, for efficiently
converting the energy of a hot gas to kinetic energy (energy of motion).
Originally used in some steam turbines, it is now used as the nozzle design in practically all rockets. By constricting the outflow of the gas
until it reaches the velocity of sound and then letting it expand again,
an extremely fast jet is produced.