Slow Neutrons
On October 22nd, 1934, Enrico Fermi decided to explore the effects of placing a sheet of lead between a neutron source and a piece of silver being irradiated with the neutrons. At the last minute, he changed his mind and placed a piece of paraffin instead of the lead sheet. What happened next was his most important discovery: Fermi noticed that the activation of the silver was much higher when the paraffin was used. It turns out the paraffin was slowing down the neutrons, giving them more time to be captured by the silver nuclei and therefore increasing the overall activation. This discovery is the basis of nuclear reactors for energy production, and was the mechanism used to produce the Plutonium of the "Fat Man" bomb.
Source: Rhodes, Richard. 1986. The Making of the Atomic Bomb. New York: Simon & Schuster.
Public Domain
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!