"What Is the Most Significant Fad of All Time?"

Some excellent answers at this link (and one dumb one), but come up with your own answer before getting your channels of memory blocked up by seeing other people's answers. And don't read the next paragraph until you've done your own thinking.

One problem is defining "fad" — and that's complicated by seeing the word significant. It seems to me that a "fad" ought to be sort of insignificant. Otherwise the answer could be: War. Or: Religion. Or: Romantic love. If you take a narrow view of "fad" and restrict the category to things that really don't matter, you may go straight to the obvious American answers: the hula hoop, pet rocks, Cabbage Patch Dolls. I think it would be best to get somewhere in the middle, like 2 of the answers at the link: cigarettes and rock and roll.

Here's the OED definition for fad: "A crotchety rule of action; a peculiar notion as to the right way of doing something; a pet project, esp. of social or political reform, to which exaggerated importance is attributed; in wider sense, a crotchet, hobby, ‘craze.'" Hmm. Seems to exclude the things I put in the narrowest definition.