Apparently this isn't a very old phenomenon at all: https://web-japan.org/trends01/article/011005sci_r.html https://web-japan.org/trends01/article/011005sci_r.h

Apparently this isn't a very old phenomenon at all: https://web-japan.org/trends01/article/011005sci_r.html https://web-japan.org/trends01/article/011005sci_r.html It seems that polishing mud balls became a trend among kids/educators during the last two decades.