I bought a ton of these sponges (they come in a large pack) from the Dollar Store. Cut each sponge in 5 strips and stack them 3 deep. Tie a piece of twine around the middle and pull tight. Be sure to double not. The kids had a blast soaking them in a bucket of water and then throwing them at each other. Lots of fun for a hot day!!
Cut in Strips and Stacked (3 Sponges per Ball):
Trim the Twine: