It is a viral campaign that has washed across the Web, and Hollywood, as celebrities dump buckets of ice water over their heads to raise money for research into amyotrophic lateral sclerosis and support for families and victims of the neurodegenerative disease.

Within a couple of weeks of its launch, the initiative has attracted Hollywood aces such as Steven Spielberg, Oprah Winfrey, Chris Pratt, Robert Downey Jr., and Tyler Perry; TV stars  Lena Dunham, Kerry Washington, Conan O’Brien, Stephen Colbert, Matt Lauer, Jim Cramer and Wolf Blitzer; singers Taylor Swift, Carrie Underwood and Selena Gomez; athletes LeBron James, Andy Murray and ‎Carmelo Anthony; and Internet billionaires Mark Zuckerberg, Elon Musk and Bill Gates. Here’s Dunham’s entertaining video of her fulfilling the challenge:

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The process goes like this. While accepting someone’s challenge, a person gets a drenched in ice water and challenges up to three others who have 24 hours to respond. If a nominee rejects the challenge, the person is asked to make a contribution to a charity. Celebrities typically do both, accepting the challenge and writing checks for ALS.

That’s what Jimmy Fallon did, when he took the challenge alongside backing band The Roots, comedian Rob Riggle, Horatio Sanz of Saturday Night Live, and Steve Higgins, the SNL writer/producer who is also announcer for Fallon’s show. Fallon was challenged by a host of notables, including New Jersey Gov. Chris Christie, New York Jets owner Woody Johnson and Justin Timberlake, whose challenge appears as part of Fallon’s video (Fallon kind of cheated in his challenge, naming the entire Jets football team):

It is not clear how the campaign started — the ALS Association says it launched last month in New Zealand as a way to raise money for cancer. US news reports say the first to do it on behalf of ALS was former Boston College baseball player Pete Frates, who suffers from ALS, also known as Lou Gehrig’s disease after the Hall of Fame baseball player who died from it in 1941. On Saturday, the association said the campaign has raised more than $10 million.

The initiative has linked friends: CBS’ Gayle King and Perry nominated Winfrey who, in turn, challenged Spielberg. It also has linked rivals, with Microsoft founder Gates (getting soaked by an elaborate contraption) nominating Facebook founder Zuckerberg and Swift challenging Gomez. Charlie Sheen did a little of both, calling out former Two and a Half Men co-star Jon Cryer, his successor on the show Ashton Kutcher and nemesis Chuck Lorre. Ben Stiller and his wife Christine Taylor nominated Judd Apatow, Ricky Gervais and tennis star Rafael Nadal. The Voice’s Carson Daly, Adam Levine and Blake Shelton did it in serial fashion (while challenging a raft of others, including Downey and Gwen Stefani):

Next in the spotlight are film heavyweights J.J. Abrams, George Lucas and Jeffrey Katzenberg, all nominated by Spielberg. The biggest name to reject the challenge so far is President Obama, nominated by fellow basketball players James and Anthony as well as Ethel Kennedy, among others. He opted to make a donation.