The Bat Jar Conjecture - S1-E13
Trivia: When Leonard, Howard and Raj are contemplating who to have as the fourth member of their Physics Bowl team, Raj says, "You know who's apparently very smart is the girl who played TV's 'Blossom'. She got a Ph.d in Neuroscience or something." The girl he's referring to is actress Mayim Bialik, who plays Amy Farrah Fowler in later episodes of The Big Bang Theory. Interestingly, she is the only recurring cast member on the show that actually does have a Ph.d in real life.