That's not entirely true. a) because kids are bad at assessing risk, even when it is obvious. Have you never seen a kid stuck somewhere they could climb up to but not down? My five year old sister managed to climb ten feet up a wall above a concrete floor, and then stayed there weakly sniffling 'help' until one of us noticed she was out of sight. b) because the old playgrounds often suffered from hidden dangers like rusting metal that could break off as splinters under the skin, which happened when I was about ten. I got a tetanus injection and then spent a week or more regularly soaking my foot in hot water to make sure any remaining rusty pieces weren't lingering in there.