First, cut a sponge into strips. Mine were multiple colors, but they certainly did not need to be. I had ideas of using the sea urchin sponges to play games with, so I wanted them to be more colorful.
Next stack them in two rows of four (see the photo below).
Then use a piece of string and tightly tie a knot around the middle of the stack of sponges. Once you tie the string tightly, the individual sponge pieces will pop up.
I originally tried using fishing line, but it was not strong enough and kept breaking on me so I switched to string.
Then left the kids dip them in paint and enjoy! My students loved painting with these sponges!