"If you hear a scream like a little girl's ... know that Jimmy Fallon is swimming in Lake Michigan," Fallon told the crowd shortly before running in.
Emanuel, wearing a green Chicago Public Library T-shirt and shorts, went first, with Fallon just after. Both were soaked as they fled the water to cheers from a large crowd. A group of bagpipers, wearing yellow rain boots and traditional kilts, provided the soundtrack for their rapid dash into the 32-degree lake.
The annual event draws several thousand hearty plungers to raise money for Special Olympics Chicago.
Scores of people dressed in parkas and polar bear outfits, some carrying signs, gathered along the lakefront early, hoping to catch a glimpse of Fallon. It was 10 degrees during the plunge, and Chicago firefighters in red wetsuits waded in before the waves of brave souls, throwing chunks of ice out of the area.
Seventeen-year-old high school senior Marilyn Lamanna and a friend got up at 5 a.m. to snag a spot where they hoped to watch the feat unfold.
With them was a large, cardboard cutout of Fallon's head, which caught his eye. He gave the shrieking girls brief hugs before darting off to take the plunge, telling them, "I've got to go meet the mayor."
"Between Jimmy Fallon and the Special Olympics, it doesn't get much better than that, even though it's super cold," Lamanna said.
Emanuel said last summer that if city's children read 2 million books as part of a Chicago Public Library program called "Rahm's Readers," he'd jump in the lake.
An hour before the plunge, Emanuel was dressed in sweats and sipping coffee. He got emotional as he talked to reporters about meeting the mother of a special needs child in a Chicago grocery store on Saturday, saying she was grateful for his participation in the event. He said they cried and hugged.
"There are parents out there, there are children out there who have something they can give," he said.
Emanuel was flanked by Detroit Lions defensive lineman and former Bears player Israel Idonije and Chicago Public Library Commissioner Brian Bannon.
Special Olympics Chicago President Jen Kramer credits Fallon and Emanuel with attracting more than 3,000 people for the run into the lake from the city's North Avenue beach. That's more than ever; last year 2,300 people did it.
Black t-shirts emblazoned with "#Swimmy Fallon"— the nickname Fallon coined in a recent tweet to the mayor — were "going like crazy," volunteer Suzy Thomas said.
And numerous Fallons dotted the crowd, thanks to masks in his likeness given to attendees, strolling around the beach quarters, moving among warming tents.
When Emanuel heard Fallon wanted him to appear on the show he took over in February from longtime host Jay Leno, the mayor called the comedian and challenged him to join in the plunge.
Fallon joked Wednesday that he agreed to it before realizing what it actually entailed. He also said Emanuel told him he had to "toughen up" before Emanuel would appear on his show. The comedian's followers egged him to do it with a #JimmyPlungeWithUs campaign on Twitter.
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