Monkey bars

“Young primates have always invited serious injury by play-climbing and falling. Primatologists view these behaviors as a critical means of fine-tuning fundamental motor skills. And despite the rise of “ultrasafe” playgrounds in the 1990s, many developmental psychologists contend that risky play is essential for healthy growth and development in children, perhaps because risk-taking helps kids regulate their fears, providing a kind of blueprint for responding effectively to real-life danger.”

https://www.smithsonianmag.com/history/history-monkey-bars-180981556/