In your falling-dominoes model of nuclear fission chain reactions, what did a standing domino represent? What did the fall of a domino represent?

Dominos are similar in that the movement of one domino causes others to fall, which is a chain reaction. The problem with dominos is that one domino usually causes one other domino to fall which is a linear chain reaction (unless the dominos are set to knock over more dominos as in a fan shape). In nuclear fission of Uranium the reaction is an exponential growth chain reaction. The fission of one atom of U235 releases two or three neutrons which can cause other atoms to split which each release 2-3 neutrons and so on. In this way one split causes 3 splits which cause 9 splits which cause 27 splits and so on. A better analogy is shown in the video where the mousetrap releases 2 balls which set off 2 traps which set off 4 traps.