The premise of the Ice Bucket Challenge offers an unusual choice: You can either donate $100 to charity, or you can film yourself dumping a bucket of ice water on your head. After which — because it’s unclear what else could be done with such a video — you can post it to social media and call upon several people to take the same challenge.

Then — boom — three things are accomplished: You have publicly shown how good you look wet, proven your charitableness and likely taken in a windfall of social media “likes.”

And those aren’t the only windfalls. The main beneficiary of the Ice Bucket Challenge has been the ALS Association, a nonprofit that raises funds to fight amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease. Donations to the organization have more than spiked — they’ve exploded. Between July 29 and Aug. 17, more than $13 million flooded the organization’s coffers, compared with $1.7 million during the same period last year, the group said in a statement on Sunday.

In a matter of days, the Ice Bucket Challenge evolved from a goofy joke between golf pros into a cultural movement. When Slate’s Will Oremus wrote a piece on it on Aug. 12, the ALS Association had raised $1.4 million. Two days later, when the Wall Street Journal published its take, the group had taken in $7.6 million. And now, the number’s up to $13.3 million.

The number of tweets likewise skyrocketed, according to Topsy, a social media analytic Web site. When the Journal report hit last Friday, Twitter users had deployed the hashtag “#icebucketchallenge” 118,000 times. But now? It’s more than 417,000.

What does it all mean? Why the ice bucket? Why Lou Gehrig’s disease? Why now?

Even proponents for ALS haven’t been able to figure it out. “It’s very difficult to fundraise because most people have never heard of ALS and it’s a very complex disease to discuss and explain,” Lance Slaughter, the ALS Association’s lead fundraiser, told the Journal. “We don’t have survivors of this disease.”

Like any good social media trend, its origins are hotly contested. But they actually have little to do with ALS. The original Ice Bucket Challenge was less specific. It asked for a charitable donation — as in, any donation. When Matt Lauer got in on it and was soaked on national TV, he did not mention ALS — and was later criticized for not mentioning an illness that at that point had no apparent connection to buckets of ice water.

Pete Frates, a 29-year-old former college baseball player in Boston who has since developed the disease, seems to have been a key player. The New York Times reported that his video of himself taking the challenge to the strains of Vanilla Ice’s “Ice Ice Baby” spurred many to get involved.

“This is a creative way to spread ALS awareness via social media and in communities nationwide,” the ALS Association said on Aug, 6.  “We thank Pete Frates and his family for getting so many people involved in spreading the word.”

And now, after two weeks, hundreds of thousands of tweets and millions of dollars, the trend shows no signs of slowing.