Here's the story behind that:

In 1939 fission was discovered – the ability to pry apart the largest atom in nature, uranium, and release millions of times more energy than any chemical reaction. Shortly thereafter a new element, even heavier than uranium, was synthesized in the lab – and named plutonium. (Plutonium's discoverer, Glenn Seaborg, jokingly suggested the symbol "Pu" for the new element and the label stuck around like a bad smell – true story.)

Not only can plutonium, like uranium, fission and release similarly vast amounts of energy – but it also happens to be manufactured in a nuclear reactor during the uranium fission process: uranium turns into plutonium, making it the only energy source that creates more fuel as it consumes itself.

Under the cloak of WWII secrecy, the Manhattan Project scientists therefore saw two routes to an atomic bomb: uranium or plutonium. With essentially infinite resources from an "all-in" US government bent on winning not just the war but also the peace to follow, they chose to do both.

Hence the two glass receptacles for the marbles: one huge, one small.

They knew that neither route would be easy. To make a bomb with uranium they needed to separate its rarest natural component, U-235, from thousands of tonnes of ore: forget about marbles – imagine filling a bucket of sand one grain at a time, with tweezers.

In a secluded valley in Tennessee, they built the largest building on earth (44 acres under one roof) and an entire town for 30,000 workers and their families – to get the job done as quickly as possible.

To make a bomb with plutonium they needed to first invent nuclear reactors (achieved by late 1942), then build the biggest reactors they could to convert uranium to plutonium, then invent a chemical process to separate the plutonium from the highly radioactive waste fuel: again, sand grains and tweezers – only now radioactive.

On a secluded plateau in Washington state, they built three behemoth reactors and a chemical separation plant, employing over 45,000 workers – to get the job done as quickly as possible.

By mid-1945 they had enough plutonium for three bombs: one to test in the New Mexican desert, and two to drop on Japan – about 6 kg of plutonium apiece. (In the end only one of these was used, at Nagasaki, before Japan surrendered.)

They had enough uranium (about 60 kilograms) for one more bomb: this design was much simpler and needed no test. The first uranium atomic bomb – and the first atomic bomb used in war – would be the one dropped on Hiroshima.

You may have noticed the factor of ten difference between the required amounts of plutonium and uranium – reflected with laudable accuracy in the film's contrasting marble collections. This can be chalked up to both engineering (the plutonium design was simply more efficient), and physics (you need much less plutonium to make an explosive 'critical mass').

For both these reasons the plutonium design concept – in ever-increasing levels of macabre sophistication – has been the path of choice for the global nuclear arms race ever since.

On a more positive note: plutonium – the element unheard of before the war – went on to achieve greatness in the cores of hundreds of power reactors around the world. In a far more pleasant role than first assigned during WWII, uranium and plutonium have been working together – the one born from the other – to keep our lights on without polluting the planet, for almost 70 years.

Today, between one-third and one-half of our nuclear electricity is from plutonium, generated in situ.

If we don't lose our marbles, this will hopefully be the only fissioning that plutonium sees from now on.