Necktie

A necktie hanging on the outside of a door is a time-honoured signal in college dormitories that the person inside is, as Penny puts it, "gettin' busy". Sheldon had no clue what the tie-on-the-door meant. Penny explained, but when he asked how he was supposed to act in response, she said that he was asking the wrong person because she was usually "on the other side of the tie".