Many years ago when people had no thermometers (温度计), they usually touched a thing to see
how hot or cold it was. The following experiment shows how easy it was to make a mistake (错误).
Take three glasses: the first one with hot water, the second one with very cold water and the third
one with water at room temperature. Then you put a finger of one hand in the hot water, and a finger
of the other hand in the cold water. You hold your fingers in the two glasses for a minute. Then you use
the two fingers, one after another, to see how hot or cold the water in the third glass is. You will find that
the water is warm to the finger that was in the cold water before, but the finger which was in the hot water will feel cold in the third glass of water.
You will often make a mistake if you take something that seems to you for a real fact. That is why
you must always find the temperature in a scientific way. You must not try to tell the temperature as it
seems (好像) to you.