Leona Woods and Enrico Fermi are walking in the snow, and the first one says…No wait, this isn't another joke setup: They're actually walking together to watch one of the most important experiments in the history of science.
Woods, Fermi, and a physicist named Herb Anderson have set up a huge pile of what look like black bricks in the middle of a squash court.
The test is to determine whether the theory behind an atomic bomb will actually work. Basically, they know that if you bombard a uranium atom with neutrons it's nucleus will split and release energy. The theory is that if you put a bunch of uranium together and bombard it with neutrons, the neutrons from the first split atoms will bombard the next uranium atoms, and then those will split even more atoms…and it will cause a chain-reaction, therefore releasing more energy than ever imagined. Thing is, they aren't actually sure this will really happen.
The black bricks are graphite blocks with small pieces of uranium slid into holes that have been drilled in the bricks. The idea is that the graphite will slow down the speeding neutrons and be more likely to collide with the uranium atoms and cause fission.
Stuck through the pile at various points are long wooden poles wrapped with cadmium—a metal with an ability to absorb huge numbers of neutrons. The idea is that as long as the cadmium poles are in place, they will prevent a chain reaction from starting.
There's a balcony (usually used to watch the squash game) where most of the scientists are to observe the experiment, but three lucky scientists are positioned over the pile holding buckets of cadmium. If the reaction gets out of hand, they're supposed a dump the cadmium on the pile and run—a kind of last minute we're-all-going-to-die delay tactic.
They slowly lift rods out of the pile while carefully calibrated machines (that go clickety-clickety-clickety, just like the old timey movies) measure the flying neutrons.
A bunch of scientists are eagerly watching the experiment, including Leo Szilard and Eugene Wigner (the guys who found Albert Einstein on vacation).
There's only one more cadmium rod left, dubbed the "zip" rod. It's George Weil's job to pull the rope that will lift it.
As the last rod clears the pile, a chain reaction begins. Fermi is psyched because it confirms all of their theories about fission.
The pile has "gone critical"—meaning the chain reaction will cause the power to double in the pile every two minutes until Fermi shuts it down.
Fermi lets the tension build by letting the experiment run a bit longer than everyone else is comfortable with, and then he has the cadmium poles put back into the pile.
All the scientists clap for themselves because "the controlled release of atomic power has been demonstrated for the first time in history."