Physics and Chemistry
The whole burden of physics seems to consist in this—from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena.
The old Candid Camera television series included an episode whereby passersby were invited to sit at a dais-type table to sample pies stacked up by the dozens. Unbeknownst to the volunteers, the two men sitting at the ends of the table were actors. One actor walked over to the other and methodically smashed his face with a cream pie. The supposed victim just as methodically retaliated. It was not long before the scene degenerated into a hilarious melee. Obviously, this scenario was a sociological counterpart to the phenomenon of a chain reaction in physics, and the critical mass, as it were, occurred when the volunteers slackened their inhibitions against assault and battery.