This is so cool! I've got to try it! Maybe the HS tennis coach will give me the used-up tennis balls. (I've taken the empty containers for armatures for papier-mache tikis, so why not the balls as well?)
I'm going to try to scavenge some balls, too. 'Tis the season. I suppose there may be some around because I notice some of our classroom teachers use them on the legs of the chairs. Follow the links from You Tube back to the original site for lots of great sketch book ideas! Amazing.