In the late 1930's, physicists noticed that when a neutron was shot into a
235
92
U nucleus, the nucleus would split into two parts, which would fly apart at tremendous speed and release two or three neutrons. They carefully measured the mass of the resulting pieces and found it to be a little bit less than the mass of the original uranium nucleus.
Following is a nuclear fission simulation:
Source: Nuclear fission, Universtiy of Colorado - Physics
When the simulation opens, use the mouse to click the red button to fire a neutron at a uranium 235 nucleus. After the nucleus is struck by the neutron, it will become unstable, and finally break apart.