It's weird, but I think they have more fun in the empty aquarium without paper shreds or anything at all in it. Last year when I had it out (for playing in snow), they had a total ball with it just jumping in and out. They get so vigorous in it that I was concerned that they were bonking their heads against it too hard! They get to wrestling so that it sounds like a pair of sneakers in a dryer! :o) Gary [Posted in FML issue 4327]