“To adults, seesaws might look like an accident waiting to happen,” said Lauren Drobnjak, a physical therapist in Cleveland and co-author of the book “Sensory Processing 101.” But “by rapidly moving the child through vertical space,” she said, seesaws provide input to a child’s vestibular — or balance regulation — system “in a way that no other playground equipment can.” And children learn strength and coordination when they hit the ground and push themselves back up. “A seemingly simple plaything actually provides so many important sensory experiences for kids,” she said. - http://www.nytimes.com/2016/12/11/nyregion/the-downward-slide-of-the-seesaw.html?_r=1