You are locked in a jail cell with no windows, and you need to tap out a message on the wall for the man in the other cell next to you. The problem is that you have to do it at exactly 9:15 PM, when the guard outside is switched, so your noise won't be noticed. You can't hear the switching of the guards through your walls, and you have no clock.
There is a faucet with water dripping very consistently from it in the corner, but you don't know if it is dripping at 30 or 40 or however many drops per minute, and that wouldn't give you the time in any case. You can just make out the chiming of a church bell, but it chimes just once at the top of each hour, so you can't tell the time from that. You can feel the wall facing west start to cool after the sun sets, but you don't know what time the sun is setting, and this isn't very precise in any case. Your dinner is always passed into your cell between 6:15 and 6:45.
How do you determine when it is exactly 9:15 PM?
