At elementary schools, kindergartens, and preschools all across Japan, kids are losing themselves making hikaru dorodango, or balls of mud that shine. Behind this boom is Professor Fumio Kayo of the Kyoto University of Education. Kayo is a psychologist who researches children’s play, and he first came across these glistening dorodango at a nursery school in Kyoto two years ago. He was impressed and devised a method of making dorodango that could be followed even by children. Once Kayo teaches children how to make these mud balls, they become absorbed in forming a sphere, and they put all their energy into polishing the ball until it sparkles. The dorodango soon becomes the child’s greatest treasure. Kayo sees in this phenomenon the essence of children’s play, and he has written academic papers on the subject. The mud balls could also offer fresh insights into how play aids children’s growth.