CBS

Season 6: Episode 8 - The 43 Peculiarity

The 43 Peculiarity: The guys realize that Sheldon disappears for 20 minutes every day shortly after lunch, so they set out to figure out what he’s doing in that time. They trail him to a secret room where the number 43 is written on a whiteboard, then exhaust themselves trying to figure out what it could possibly be. Later, it's revealed that Sheldon uses the 20 minutes to mentally decompress by playing hackey sack. Leonard gets jealous when Penny starts texting a British guy in her class, so he tries to scare the guy off. Penny eventually tells him she loves him, which calms him down, but then Sheldon's assistant Alex starts flirting with him.