At a certain point last week the knowledge that people I knew were dousing themselves with buckets of ice water became inescapable.

Other things that have gone viral this summer include #yesallwomen, a Kickstarter for potato salad, and a donation fund for a police officer who shot and killed an unarmed teenager. How they’ve done this is mysterious — while there are entire ad agencies devoted to “viral advertising” and you can pay your way into the “trending” section of Twitter these days, all of the above seemed to spread without any Machiavellian strategy behind them.

What made the ice bucket challenge stick, when so many other fundraisers and attention-getters haven’t?