Improvisers have always been the daredevils of theater, leaping onstage without a clue what they're going to do once they get there. Always looking for the next big thrill, the higher high, the more dangerous stunt, many of them took to long-form improvisation, creating a 30- or 40-minute show based on a single suggestion. A marathon improv is just the next step. Seasoned improviser Andy Cobb has gathered a handful of other improv fiends for such an event, among them Mick Napier, Ed Furman, Craig Cackowski, T.J. Jagodowski, Lisa Brooke, and Keegan Michael Key. Many are veterans of other improv stunts, including the Annoyance Theatre's notorious Donkey Improv, in which a live farm animal was one of the players. Cobb's premise is much less extreme: he'll take one suggestion from the audience, then the ensemble will improvise for 24 hours straight. Six other improvisers will pop onto the set from time to time to freshen things up, but the original seven will remain onstage the whole time. Yes, Cobb assures me, people will get potty breaks. But anyone who dozes off will have his or her nap integrated into the show. The event is a fund-raiser for a 24-hour homeless shelter in Uptown, Residents for Effective Shelter Transitions, but the charity is probably less important to these guys than the thrill. The Second City E.T.C., Piper's Alley, 1608 N. Wells, 312-337-3992 or 877-778-4707. April 15-16: Monday, 10:30 PM, through Tuesday, 10:30 PM. $10 for "anytime" admission (the event also includes a party, raffle, and live music).