What is a structure and how do you make it sturdy? Hamza wants a new bookcase to replace his rickety old one. He enlists the help of an apathetic beaver to build it and learns that there are three forms that are the basis of all structures: frames, shells, and solids! We learn: the meaning of frames, shells and solid structures, and where we see them in nature and everyday life; some objects like a guitar are combination structures; concrete can be made lighter using expanded glass aggregates instead of heavy gravel.