The Big Bang Theory Season 4 Episode 3 : The Zazzy Substitution

Sheldon and Amy Farrah Fowler are very compatible with each other, as evidenced by how well they can play "counter-factuals", a game they invented requiring them to know what the other is thinking when answering a question about "an alternate world that differs from ours in one key aspect" (Leonard flunks miserably when he attempts to play this game). Bringing Amy into the group is a source of consternation, as the guys can barely handle Sheldon ...