You're in a room with 3 light switches. Next to your room is another room with the door closed. Inside that room are 3 lights. You do not know which switch controls which light. How can you figure out which switch controls which light? You can only enter the room with the lights once.
Turn one switch on and leave it on for 1 hour. This is switch A. After the hour is up turn the light off and turn another light on (switch B). Go in to the room with the lights. The light that’s hot is light A. The light that is on is light B. and the light that is off, but not hot is light C.
He who builds me doesn't want me,
he who buys me doesn't use me,
he that uses me doesn't know he's got me.
What am I?