A team of electricians was refurbishing an old underground train station and at the end of the day all of them had gone home apart from Fred, who was just finishing in the control room. On the wall above the desk were three switches, marked A, B and C, that operated three incandescent light bulbs on the Northbound platform three levels below.

One switch operated a red bulb that instructed trains to stop, another operated a green bulb that told them to go and a third was for an amber bulb that told the train drivers to get ready. All the bulbs were mounted on a black panel by the edge of the platform. Fred's colleague, who had now gone home, had left a note to say that he had checked the bulbs and they were all working fine. The trouble was, although the switches were marked A, B and C, his colleague had forgotten to make a note of which switch operated which light. The switches had been rewired and needed to be labelled, STOP, WAIT and GO.

Before he left for the evening himself, Fred wanted to finish the job for his colleague, but the only problem was that the lift down to the Northbound platform would operate only one more time that day. He could turn on as many of the switches as he wanted but he could visit the Northbound platform only once to see which lights they operated. Switching on all the lights would obviously tell him nothing and if he turned on only one light he would have the answer for only one switch as he could visit the platform only once. Despite this, by using logic and a bit of lateral thinking he was able to work out which switch was for each of the three lights.

There is no trick to this. Fred visited the Northbound platform only once, and after doing so he knew exactly which light bulb was operated by each of the three switches. There were no CCTV cameras and no one was down there to tell him over the phone. Can you work out how Fred did it?

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This is another variation of an old classic and many of you may have come across this in one of its many different forms. For those who haven't here's the solution. Fred turns on two of the lights, let's say A and C, and then goes and makes himself a cup of tea. When he returns he switches off C and then goes down to the platform. The bulb that is still on will be A and the bulb that is off but warm will be C.