There is a corridor that leads into an empty room. the room is completely empty except for a lamp in one of the corners. on the other side of the corridor are 3 switches. you know that one of them turns on the lamp, the other 2 don't do anything. you can play around with the switches turning each of them on and off as much as you like, then you can go into the room 1 (only one!) time to check whether the lamp is on or off (there is a door so you can't know without entering the room). leaving the room, you have to know which of the 3 switches works and which 2 don't.
Here's a riddle my friend posed to me today. I don't want to look up the answer since that would be cheating (and cheaters never win), but if somebody were to figure it out and maaaaaaybeeee leave a little comment describing how said riddle miiiiight be solved.... then that'd be okay. Here's the riddle: