BURBANK, Calif. — "The Big Bang Theory" bucks two current trends in television. First, it's a sitcom that's really, really funny. And, second, it's a sitcom that's really, really successful.

And there aren't many sitcoms that can make either of those claims anymore.

Co-creator/executive producer Chuck Lorre rejects the idea that he knows something very few others do these days, but the fact is that he's got two successful sitcoms on the air (including "Two and a Half Men") at a time when whole networks don't have even one.

"We just make shows that we love to laugh at ourselves and hope that somebody else out there agrees," Lorre said. "That's really about it. If it's not funny to us, we don't do it."

"I'll tell you this," interjects Bill Prady, the other "Big Bang" co-creator and executive producer. "The pleasure and pain of working with Chuck is that Chuck, every minute, on every script, on every page, in every moment, says two things: 'Do we believe the characters would do this?' and 'Why are we laughing?' "

The simple answer to why America is laughing at "The Big Bang Theory" (Mondays, 7 p.m., CBS/Ch. 2) is that it's funny. Like so many successful shows, the premise is actually quite simple — a variation on the classic fish-out-of-water story.

Leonard (Johnny Galecki) and Sheldon (Jim Parsons) are brilliant physicists/roommates who struggle with the real world. Sheldon, in particular, can't comprehend how "normal" people operate.

They hang out with their buddies/fellow geniuses, Howard Wolowitz (Simon Helberg) and Rajesh Koothrappali (Kunal Nayyar), playing and Klingon Boggle. But when a beautiful blonde, Penny (Kaley Cuoco), moves in across the hall, Leonard wants something more. Maybe even a relationship.

"Leonard is the only character that's in motion by his own choice," Prady said. "He is the only one who is reaching for something."

"That was apparent to me early on — that he just had this nagging notion that other people have fuller lives than him," Galecki said. "He just doesn't know how to attain that. There's no manual, and he has not been given an invitation outside of the world that he lives in, and it's frustrating. So there's this kind of fun dichotomy to play — that frustrated impotence, and yet he's a genius."

The show's breakout character, however, is Sheldon. He's often insufferable, but he's utterly without malice.

"There's an innate charm and sweetness to Jim that it allows us to make him as obnoxious as we want, and we can get away with it," Lorre said. "There's an innocence to it that comes through."