yeah cardboard... we had made sort of a castle for the cats out of 5 different cardborad boxes. At first they were playing with it a lot, but then, they just started biting it to pieces. There were pieces of cardboard box everywhere horrible to pick them up all the time. So, we had to take it away. Now they try to find all kinds of other stuff to bite, like emptying everything we have on our comp table... who cares about the toys we have for them