Apprently it is a bit "zzzzz" on here or something. Try your Brain at this:
Acting on an anonymous phone call, the police raid a house to arrest a suspected murderer. They don't know what he looks like but they know his name is John and that he is inside the house. The police bust in on a carpenter, a lorry driver, a mechanic and a fireman all playing poker. Without hesitation or communication of any kind, they immediately arrest the fireman. How do they know they've got their man?
Okay, maybe that was easier than I thought - maybe I need more coffee. Try this one:
There are three switches downstairs. Each corresponds to one of the three light bulbs in the attic. You can turn the switches on and off and leave them in any position.
How would you identify which switch corresponds to which light bulb, if you are only allowed one trip upstairs?
I would turn one switch on, leave it on for a bit, turn it off and turn another on and move to the attic quick.
The bulb that's off, but is still warm goes to the first switch, the bulb that is on goes to the second switch, the bulb that is off goes to the final switch.