George Clooney is famed for pranking his fellow actors on set—something Matt Damon learned firsthand.

George even tricked his The Monuments Men costar into thinking he was gaining weight during filming. But, as Matt explained Tuesday, Jan. 14, on The Tonight Show With Jay Leno, George was so stealth in his handiwork that he didn't even realize he was being pranked until after the movie wrapped.

"I read finally, which answered a lot for me, that he'd been taking my wardrobe in," said Matt, explaining that "every couple of days" George was having his pants taken in "about an eighth or a sixteenth of an inch."

VIDEO: See George Clooney and Matt Damon inThe Monuments Men trailer

This caused a lot of confusion for the father of four "because [George] had said, 'We're gonna get in shape!'"

"I was working out, and the pants [were getting tighter]—I said, 'This just can't be,'" Matt, 43, said with a laugh. "I never said anything to him, which he probably loved even more."

Indeed, George, 53, loves having the upper hand when pranking his pals. "I do enjoy whenever an actor's trying to lose weight having the wardrobe people take in their clothes," he admitted to E! News in November at the BAFTA Los Angeles Jaguar Britannia Awards,  "It just makes me laugh, watching them eating like a grape the next day…And they do it for weeks. It's good fun."

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