Question: what is the difference between fat man and little boy?
Answer: It really boils down to the fact that Little Boy (the first atom bomb used in warfare) used Uranium-235 as filling and had a blast yield of 15 kilotons of TNT and was detonated over Hiroshima, while Fat Man used Plutonium as the filling and had a blast yield of 21 kilotons of TNT and was detonated over Nagasaki.