You are standing next to three switches. You know these switches belong to three bulbs in a room behind a closed door – the door is tight closed, and heavy which means that it’s absolutely impossible to see if any bulb is on or not. All three switches are now in position off.
You can do whatever you want with the switches and when you are finished you open the door and go into the room. While in there you have to tell which switch belongs to which bulb.
Yay, I can do this one! You switch one switch on for a little while, then switch it off, then switch the next one on & open the door. the bulb that is lit is the switch that is on. the bulb that is still warm is the one you just switched off and the remaining bulb is the last switch.