Crittertrail Fun-Nel Tubes Attach Directly To Each Other And To The Inside And Outside Of Your Crittertrail Home. If You Can Imagine It, You Can Build It With Crittertrail Fun-Nels! The Tubes Are Available In Four Colors And In A Variety Of Shapes And Sizes So You Can Create A Huge Playground For Your Pet. Our Fun-Nel Elbows Are Perfect For Creating Twists And Turn Connections To Your Critters Maze Of Fun. With Crittertrail Accessories, Your Imagination Is Your Only Limit.