It's a trip to the past on 'The Big Bang Theory' (Mon., 9:30PM ET on CBS). The show flashes back seven years, so that we can see how Sheldon (Jim Parsons) and Leonard (Johnny Galecki) met and became roommates.

[Spoiler alert.]

In addition to telling a story from the past, the episode also takes on the important issue of time travel. After Sheldon and Leonard meet for the first time, they create the world's most bizarre roommate agreement. Part of their mutual deal: if either of them invents a time machine, the first thing they will do is go back in time and appear in the room within the next five seconds.

They sign the agreement, then wait eagerly for their future selves to show up in the room. Five seconds tick past. Nothing happens. "[Well,] that's disappointing," says Sheldon.