The Double Slit Experiment
No, it's not the Olsen Twins first venture into pr0n, it's the foundational experiment that led to modern quantum theory. Unfortunately, the video ends before getting to the coolest possible conclusion to be made from the experiment: the Many Worlds Interpretation.
The phrase "many worlds" was first used by Bryce DeWitt, who wrote more on the topic following Everett. In each of these worlds, everything starts out identical, except for the one initial difference; but from this point on, they develop independently. No communication is possible between the separate universe, so the people living in them (and splitting along with them) would have no idea what was really going on. Thus, according to this view, the world branches endlessly. What is "the present" to us, lies in the pasts of an uncountably huge number of different futures. Everything that can happen does happen, somewhere.
The best fictional depiction of "the Multiverse," in which all of the potential Universes overlay one another simultaneously, is Stephen Baxter's "Manifold Time" - great, great read. Also well worth your (less than manifold) time is his brilliant sequel to HG Wells', "The Time Machine," called, "The Time Ships."