This is the most fantastic invention!  We always kept our gerbils in an aquarium tank as they chewed through anything else and had to exercise them by letting them run around in the bathtub, until now.  They are so happy, they have so much room in the bottom now to burrow and make tunnels because the food, water and exercise wheel are in the extension.  I love it, can't understand why it's not available in any of the pet stores in Canada.