We all have an innate drive to play, and this impulse is one of our most natural and effortless sources of joy. And the most playful shapes, the ones found in hula hoops and beach balls, carousels and merry-go-rounds, are circles and spheres. Research shows that a part of the brain called the amygdala, associated in part with fear and anxiety, lights up when people look at angular objects but stays quiet when they look at curved ones. In a manmade world defined by endless right angles, rounded shapes set our minds at ease and invite our playful inner child out into the open.