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Sensational - the try 'em at home guide to illusions

The Temperature Illusion

Hot-cold running water

Take 3 glasses of water, put one in the fridge, fill the other with warm water from the hot tap (just warm NOT hot water) and fill the third with normal cold tap water.

Put the three glasses in a row in the order: warm, normal and cold. Pop a finger into the warm water and, with your other hand, pop a finger in the fridge-cold water. Leave for a short time, and then put both fingers into the middle glass. You will feel that the finger that was in the warm water feels cold and the finger originally in the fridge-cold water now feels warm, even though they are both now in the same temperature water. What's happened here is that when your finger was, say, in the warm water it adapted to that temperature. Your body is really only interested in things that change in the environment, as it is the changes that you need to be able to react to. So, after your finger has adapted to the warm water when it goes into the middle glass it's 'expecting' it to be warm. When it's not your brain reasons, "this isn't warm so it must be cold". Similarly, the finger in the fridge water adapts to coldness, and when it moves into the centre glass, that's not cold so it must be hot. So one finger is saying hot the other is saying cold and both are actually at the same temperature.