http://stores.ebay.com/BluestoneCommerce-Cages-and-Guitars Coroplast is not cardboard nor is it plastic-coated cardboard. It is an extruded twin-wall polypropylene plastic sheet product. In other words, it's 100% plastic. This material was originally chosen years ago for guinea pig cage application by a guinea pig rescue organization as a way of economically constructing large living quarters for rescued pigs. It is lightweight and durable and, if not abused, will last for years in your guinea pig cage. View the video to learn all about Coroplast and see a dramatic demonstration of it's strength. Visit our blog: http://www.bluestonecommerce.blogspot.com/