Another ping pong ball on another mousetrap: Israel Tells U.S. It Will Retaliate if It Comes Under Attack by Iraq. For those who never saw it in science class, there was a movie which explained nuclear reactions by showing a bunch of mousetraps set with pingpong balls on their springs. If there were enough traps close enough together (critical mass), then a single ball tossed in from outside would spring a trap, and its released ball would spring another, and soon all of the traps would be sprung (chain reaction).