Most children think that light things float and heavy things sink--but what do you do with a battleship? Mr. Branley identifies the problem very clearly but doesn't solve it as adequately. The topic may even be a little difficult for the young reader--psychologists use it to determine when young adolescents are capable of formal logical thinking--but the presentation is too immature for the older child. The colorful illustrations and suggested activities are quite good, adding to the perception of the problem, and then it is left hanging--the relational concept of density has been nibbled at but not established. In this it resembles Big Tracks, Little Tracks and one wonders about the purpose of both books; in either case, aiming at an older audience would have enabled the author to identify the larger context.