My 4-year-old son, Theo, goes to an outdoor preschool. For two mornings a week, he rambles through woods that he shares with great horned owls, black bears, and white-tailed deer. The number of nature-based educational programs in the U.S. has increased 10-fold since 2012. There are good reasons the idea is catching on. I wrote for Outside about the benefits of bundling your kids up and sending them to school, outdoors.