In the basement of a house there are four incandescent bulbs and on the top floor there are four switches, one for each bulb. When we operate a switch from the house, it is impossible to see which bulb has been lit.
The question is: Making a single trip, How can we know which switch turns on each light?.
The most logical method is to press two switches for a while, and then turn off one of them and turn on a third. When you go up there will be a light bulb turned off and cold (the one corresponding to the switch that has not been touched), another one turned off and hot (the one that turned on for a while and then went out), a light and not very hot (the one that has just been turn on) and another on and very hot (the one that has been on all the time).