Just when we thought playgrounds were accident-proof -- no more merry-go-rounds, high slides, jungle gyms, seesaws or pretty much anything that's fun -- it turns out that safety itself can be dangerous. A recent heat wave in New York exposed a new playground risk: The ubiquitous rubber safety matting gets hot, not as hot as McDonald's coffee, but hot enough to scald tender feet.

The outrage was immediate. "Playgrounds should be designed with canopies," one park- safety advocate declared. "How many burn cases will it take,"...