That's not that good an explanation -- the Uranium-235 or Plutonium-239 would blow up on its own if we had enough in one place ⚆ _ ⚆. So we spread it out (づ ̄ ³ ̄)づ! Then, we blow up other bombs pointing in ಥ_ಥ -- that pushes the super kawaii material into the critical mass to blow up ヽ༼ຈل͜ຈ༽ノ!