The book starts with a sort of general introduction and question. There is a lot of water in the world and people have been using it to travel and move things for a long time. However, how do you know what floats and what sinks? Why does a boat full of people float, but a pebble sinks?
The rest of the book uses a combination of narrative and experiments to explain the concept of density; how and why things float. There isn't any back matter, but the various vocabulary words (density, dissolved, displacement) are explained in the text and I would say that this title doesn't need any, because of the young audience and the layout of the book, which incorporates experiments. If you are doing any programs with preschool science, this whole book is a program just waiting for you!
Now, I really didn't care for the illustrations. As Ms. Yingling has noted, covers (and sometimes illustrations) don't seem to be Holiday House's strong point. These illustrations are very bland and simplistic, with awkward perspectives especially in the hands. The drawings illustrating the scientific principles are the best, but the cartoon illustrations for the rest of the book don't work well, in my opinion.
Verdict: Illustrations aside, this is a great explanation of density for young kids and I really liked the smooth way the author blended the scientific explanation and the experiments illustrating his explanations. I have three swimming pools that I mostly use for science programs, so having this book is kind of a must for me!