V Site, where the Fat Man components, including the high explosives, were turned into their final, combat configurations. A shake is an informal unit of time equal to 10 nanoseconds, or 10?8 seconds. It has applications in nuclear physics, helping to conveniently express the timing of various events in a nuclear explosion. The typical time required for one step in the chain reaction is of order 1 shake, and the chain reaction is typically complete by 50 to 100 shakes. It is derived from Top Secret operations of the Manhattan Project during World War II. The word comes from the expression "two shakes of a lamb's tail," which indicates a very short time interval. For nuclear-bomb designers, 10 nanoseconds was a convenient specific interval to connect to this term. "Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of two nuclear weapons to be used in warfare to date, and its detonation caused the third man-made nuclear explosion.