You walk up to the urinal, or step into a busy airport restroom, or close the narrow door of an aeroplane lavatory. You need to go. You have needed to go for a while. And nothing happens. The harder you focus, the more your body seems to lock. Footsteps approach, a tap runs, someone shifts their weight in the next stall, and the window closes completely. You give up, wash your hands, and walk out still needing to go.