Well, technically they are dense clumps of a species of filamentous green algae called marimo, but mossy balls sounds so much classier. Mine are roughly the size of ping-pong balls, they usually clump together at the bottom, but when I change the water I wring them out so some float for a bit before sinking. They also will rise and sink as small “pearls” of air produced by the algae cling to the surface of the ball.

Marimo are popular in Japan, where they are given as gifts, and are becoming increasingly popular in the US. Since they are possibly the easiest plant I’ve ever taken care of, I can see why.

My sister gave me one, the rest came from eBay.