In free play, ideally with kids of mixed ages, the children decide what to do and how to do it. That's teamwork, literally. The little kids desperately want to be like the bigger kids, so instead of bawling when they strike out during a sandlot baseball game, they work hard to hold themselves together. This is the foundation of maturity.
The older kids, meanwhile, throw the ball more softly to the younger ones. They're learning empathy. And if someone yells, "Let's play on just one leg!"—something they couldn't do at Little League, with championships (and trophies!) on the line—the kids discover what it means to come up with and try out a different way of doing things. In Silicon Valley terms, they "pivot" and adopt a "new business model." They also learn that they, not just grown-ups, can collectively remake the rules to suit their needs. That's called participatory democracy.