In nuclear physics terminology, first you need to hit the barn, and next you need to wait for 50 to 100 shakes. And then the bomb blows up.
The Manhattan Project, which created the first nuclear weapons, was obviously top secret, and that meant code-words. This went as far as the units of measurement used by scientists working on the project, and two in particular are still in use today: the barn, and the shakes.
A barn is a unit equivalent to 10−28 m2. It was coined in the Manhattan Project because physicists were trying to bounce particles off uranium to set off a chain reaction. Uranium is a pretty big particle, so the process was like trying to “hit the broad side of a barn.” The name stuck.
Once you’d hit the metaphorical barn, nuclear fission begins. Uranium 235 hit by a neutron releases a lot of energy and two or three more neutrons. If one of those neutrons hits another uranium 235 isotope, it too releases energy and neutrons, which may then hit more uranium… and boom, you have a nuclear chain reaction.
This happens very fast: each reaction in that chain takes about 10 nanoseconds. The physicists of the Manhattan Project call that time a shake. You could reliably predict how destructive a bomb would be based on the mass of fissile material in the core and the number of shakes since the first neutron bombardment. A 10kg core, for example, would take 57.9 shakes to reach the destructive power of 100 kilotons of TNT.
Why did they call it a shake? Well, 10 nanoseconds is not very long – pretty close, in fact, to “two shakes of a lamb’s tail.”