In his first children's book, a British physicist demystifies the theory of relativity by having Einstein appear as a kindly uncle doing gedanken (""thought"") experiments. ""Uncle Albert's"" young ""relative,"" Gedanken, proves that all things are relative--and that only the speed of light is constant. Since all the neat topics (like dinosaurs) have been taken for Gedanken's science project, her uncle Albert agrees to help by thinking Gedanken into his ""thought bubble,"" in which she goes on a spaceship to chase a beam of light that speeds away saying ""catch me if you can."" She can't; and when she returns, her watch is 40 minutes slow! Uncle Albert deduces that when Gedanken approached the speed of light she got heavier, and that light goes at the same speed regardless of the observer's speed--which means that time slows down, too, and that nothing is fixed: a spaceship moving away from the earth is the same as a fixed spaceship with the earth receding (She gets a B.) Dragging in narrative hooks such as school dances may seem irrelevant but will help to hold attention, as will the breezy style. A commendable effort to lighten an important subject, though finding the ideal readers may be a challenge. Bibliography.