Most neutrons produced in fission are "prompt", i.e., created with the fission products in less than about 10 nanoseconds (a " shake " of time), but certain fission products produce additional neutrons when they decay up to several minutes after their creation by fission. These delayed-release neutrons, a few percent of the total, are key to stable nuclear reactor control. Without delayed neutrons, in a reactor that was just barely above critical, reactor power would increase exponentially on millisecond or even microsecond timescales – much too fast to be controlled with current or near-future technology. Such a rapid power increase can also happen in a real reactor when the chain reaction is sustained without the help of the delayed neutrons. This is prompt criticality, the most extreme example of which is an exploding nuclear weapon where considerable design effort goes into keeping the core deep into prompt criticality for as long as possible until the greatest attainable percentage of material has fissioned.