Sheldon has a particular door knocking rite when alerting someone to his presence outside their doorway. Due to his obsessive compulsive tendencies, he must finish his knock, even if the person or persons he’s alerting opens the door before he is finished, in which occurrence he will usually finish it on the door frame.
His knock consists of three knocks before stating the person’s name, and then he recurs this process two additional times 😛 TAGEND
( knock, knocking, knocking) “Penny ?”
( knocking, knocking, knocking) “Penny ?”
( knock, knock, knocking) “Penny ?”
This knocking is mostly associated with his trips to Penny’s apartment across the hallway, but is his general knocking 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.