Season 6, Episode 8 - The 43 Peculiarity
Aired November 15, 2012
Howard and Raj are intrigued when they notice Sheldon disappears every day at 2:45. When they secretly follow Sheldon down to the basement, they are made even more curious by the number 43 written on a blackboard. Meanwhile, Leonard gets jealous when Penny invites a guy in her history class over to study.