Again, not what you think.

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.

"Kayo first became interested in dorodango in May 1999. As part of his research in developmental pshychology, Kayo visited a Kyoto preschool once a week. When Kayo made mud balls with the children, a teacher at the preschool told him, "I'll show you a real dorodango and proceeded to produce a shiny one for Kayo"

Sources
Bits & Pieces. How to make a shiny ball out of mud
Trends in Japan. Kyoto Professor Taps into the Essence of Play