The critical mass of fissile material is the amount needed for a sustained nuclear chain reaction. It depends upon the material, its shape, its purity as well as whether it is surrounded by a neutron reflector. In the case of a sphere surrounded by a neutron reflector it is about 15 kg for U-235 and 10 kg for Pu-239.
Until detonation is desired, a nuclear weapon must consist of a number of separate pieces, each below the critical size either because they are too small or unfavorably shaped. To produce detonation, the fissile material must be brought together rapidly.
See also Nuclear weapon design.
For the car-free environment event, see Critical Mass.