Uranium 235, or U-235 for short, is radioactive. This means each molecule puts out a lot of neutrons. These neutrons fly off in random directions, so they are coming out like light rays off a light bulb. Neutrons are small particles that together, become molecules. A molecule contains neutrons, protons and electrons.
The flying neutrons eventually hit another molecule of U-235 because all the molecules are packed close together. When it does, it splits the hit molecule in two. One half of the split molecule becomes barium-141 and the other half becomes krypton-92. In the process of splitting, the hit molecule also gives off three neutrons.
The three neutrons that flew off from the split molecule also eventually hit something. That something is three other new molecules. In turn, these three molecules also split in two, giving off three more neutrons. Now there are 27 neutrons flying free, three from the original split and three from the new split. Each new split also produces three neutrons. Therefore three times three times three equals 27.
Every time a molecule is hit, it puts forth three neutrons. The neutrons keep multiplying by three since split molecules put forth three neutrons. In a short time, 27 splits becomes 81 splits, eighty-one splits become 243 splits, and so on. A chain reaction becomes free running, and if not controlled, an explosion will result. Fortunately, steps are taken to prevent this. U-235 fuel is shaped in long pencil-like rods. When the splitting has to be reduced, the rods are moved farther apart so the neutrons do not hit as many molecules.