"Cao Chong is an elephant" is a story almost known to women and children in China. Cao Chong, who is only six years old, solved a big problem that even many learned adults were at a loss by using the physical principle that the gravity of an object floating on the water is equal to the buoyancy of water on the object. This is a miracle.

In fact, the clever Cao Chong's method is "equal replacement method". Using many stones instead of elephants, marking on the ship's side, making elephants and stones have the same effect, weighing stones repeatedly, and transforming "big" into "small", this problem can be solved satisfactorily.

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Equivalent substitution method is a commonly used scientific thinking method. Here is a short story of Edison.

The great American inventor Edison had an assistant named Aptom, who had a good foundation in mathematics. Once, Edison gave Aptom the glass shell of a light bulb and asked him to calculate the volume of the light bulb.

Aptom looked at the pear-shaped bulb shell, thought for a long time, drew the sectional view and perspective view of the bulb shell, drew a series of complicated curves, measured the data one by one, and listed a series of formulas.

After several hours of intense calculation, the result hasn't come out yet. Edison was very dissatisfied after reading it. I saw Edison fill the bulb shell with water and then pour the water into the measuring cup. In less than a minute, he calculated the volume of the light bulb. Here, Edison replaced the volume of the bulb shell with the volume of water poured into the measuring cup, and also used the equivalent replacement method.