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Because of their shapes. Fat Man used plutonium. A sphere of plutonium was surrounded by conventional explosives which were designed to send the force of their explosion inward to crush the sphere into a smaller shape (thus higher density) in order to trigger the nuclear reaction. Thus the bomb had a very rounded shape to it, suggestive of a fat man. In contrast the other bomb, which used uranium, had the nuclear reaction triggered by firing a small slug of uraniun down a track into a larger mass of the same element. Thus that bomb looked long and slender by comparison and so picked up the moniker Little Boy in contrast. Michael Montagne

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10y ago

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