Came across a fascinating anecdote recently about the allies interrogating Werner Heisenberg after WWII.
It seems that the reason Hitler shelved Germany's heavy water experiments, late in WWII was that the Nazi scientists leading his heavy water experiments did not know that a certain amount of fissionable material would cause a chain reaction and trigger an atomic explosion.
Since Hitler and his scientists thought that tons of uranium might be needed, they instead focused on the V-1 and V-2 projects instead. It turns out that about 20 pounds of uranium or 10 pounds of plutonium is needed (a lot to be sure, but not tons).
The way we learned this information was that as Heisenberg's interrogation began, he asked his interrogator a question (yes he was supposed to be answering them I know). He wanted to know what size ship had delivered the American atomic devices to Hiroshima and Nagasaki and specifically wanted to know if carriers had been the ships to deliver them (the largest ships in the American navy).
When Heisenberg was told that it was not a ship that delivered the device, but a B-29 bomber, he then (knowing the size of the bomb bay for a B-29 bomber) did the math in his head and arrived at the size of the bomb (keep in mind this was while being interrogated).
Anyway- I found this story neat so I pass it along to you.