When they were constructing the pile at the University of Chicago, they had to assemble tons of uranium into stacks, or piles. That's where the name Atomic Pile came from. The uranium slugs were delivered in wooden crates which the workmen neatly stacked on the side of the room. One day when Fermi was touring the site, one of the workmen commented to him that the stack of crates resembled the actual pile. Fermi turned white and immediatly pulled out his slide rule and started furiously calculating. After a few minutes he was sure of his results and relaxed. It had occured to him that with all of the uranium stacked so close toger in the crates, it was possible that a critical mass could be achieved. This would have started a chain reaction like the one that powers a nuclear reactor, which would have been a bad thing(tm). His calculations however made it clear to him that the wood in the crates would act as an efficient neutron regulator so that no such event was possible. None the less he requested that the workers space the crates out around the room.