His knock consists of three knocks before stating the person's name, and then he repeats this process two additional times: (knock, knock, knock) "Penny?" (knock, knock, knock) "Penny?" (knock, knock, knock) "Penny?" This knock is mostly associated with his trips to Penny's apartment across the hallway, but is his general knock for any character in the series. Sheldon will also use this knock on the wall above someone's head if he enters their bedroom and is going to wake them up.