system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are bombs (especially the atomic bomb during the Manhattan project) built safely without worrying about it going off?

Manhattan project specifically since it was the first known time the atomic bomb was being created here in America, so research/knowledge was scarce.  
  
Edit never thought this would...*blow up*. Thanks for the awesome responses guys!
gpt
uranium and plutonium aren't like gunpowder. It's not like a stray spark would make them explode. They're arguably safer to work with than conventional explosives in most respects.

Nuclear material will only actually explode under very specific conditions. Building a machine that actually was able to create the heat and pressure necessary was the whole second half of the manhattan project (the first half was isolating enriched uranium). The bigger worry was that the bombs wouldn't explode at all.