Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. You can learn a lot about a society from the way they raise children. That includes not only what children learn, but how, when, and where they play. Our modern concept of childhood emerged during the Enlightenment movement of the 18th century. Influential figures like philosopher Jean-Jacques Rousseau promoted the idea that children need special opportunities to explore and express themselves through playtime.  Before then, children were treated essentially as small adults. Of course, kids in ancient or medieval times liked to run around and play as much as kids today, and they did so wherever they could. But only after childhood became thought of as a distinct stage of life with unique needs did adults start to design spaces like playgrounds. When we look at the history of playgrounds, we can see how ideas about children’s play have changed over time.…