You can learn a lot by building a rock wall. This six-year-old spent almost the entire day building and refining this wall. He selected stones, hefted and carried them, occasionally asking for help from classmates. He filled the crevices with dirt to make it more steady. Several times he watched much of his work collapse. After shaking his fists and growing frustrated, he got back to work. He refined his methods. And at the end of the day, he took pride in his accomplishment.
This is free play. No one asked him to build that wall. He could have spent those hours twiddling his thumbs. But why would he do that? Children are far superior at engaging themselves in meaningful activity than most adults. And nine times out of ten it is us, the adults, that pull our children away from the education they so eagerly seek on their own.