You can never be quite sure when you put up one of those ‘Do Not Disturb’ signs that you will not, in fact, be interrupted. To get rid of that nagging doubt that lingers in the back of your mind there is really only one perfect solution: prevent someone from entering, physically. One clever way to be sure you are not bothered in your room is to have a retractable door handle.
http://dornob.com/retractable-do-not-enter-door-handle-ensures-privacy/
