Tom had discovered the great law of human action. What was it?

In Chapter Two of The Adventures of Tom Sawyer, Tom is forced to whitewash a fence. The fence is thirty yards long and made of boards nine feet high. Since it is a Saturday, all the other boys would be going off to have fun. Tom is smarter than the other boys and manages to get them to do his work for him by pretending that whitewashing a fence is not really work but pleasure. He soon has the boys asking him to let them share in his pretended enjoyment. But he refuses at first on the grounds that it takes special skill. In the end they not only do the whole job but actually pay him for the privilege with marbles, toys, and all sorts of whatnots. 

He had discovered a great law of human action, without knowing it—namely, that in order to make a man or a boy covet a thing, it is only necessary to make the thing difficult to attain.

The narrator goes on to explain that work is something that a person is obliged to do, while play is something a person is not obliged to do. Proof of this assertion is the fact that a number of boys get pleasure out of whitewashing 270 square feet of board fence. They actually apply three coats of whitewash, so the total area painted would amount to a little over 700 square feet. Tom's Aunt Polly is amazed when she comes outside to inspect the job.

When she found the entire fence whitewashed, and not only whitewashed but elaborately coated and recoated, and even a streak added to the ground, her astonishment was almost unspeakable. 

This is perhaps the best-known episode in all of Mark Twain's writings. Most editions of The Adventures of Tom Sawyer contain at least one color picture or sketch of a small barefoot boy painting a fence with a big brush. It seems symbolic of small-town boyhood in America of the nineteenth century.