This review of dinosaur extinction theories examines clues and possibilities, heightening interest as we get deeper into the mystery. Disposing early of the theories of ""old age,"" poisoning by new plants, and predation on eggs, which don't explain the sudden extinction of all prominent Cenozoic animals and plants, Branley spells out why large dinosaurs, even if warm-blooded, could not survive the cold temperatures mammals tolerate, then shows how ""clues from plankton"" and ""clues from pollen grains"" point to a sharp temperature drop at that time. There is a similar ten-page examination of the phenomenon of drifting continents and its possible role in the extinction. Branley considers the supernova theory, then its shortcomings, and deals more hospitably with the theories of asteroid collision and volcanic eruption. Finally he takes up the ""ring"" theory--that a ring of dense ash cloud formed around the equator, making winters colder and longer but not interfering with summer sun--which would explain why certain animals and plants survived and others didn't, and which is consistent with both the asteroid and volcano theories, whether the collision or eruption occurred on earth or the moon. Simply as an answer to the subtitle's question, this is better informed, better reasoned, and more up-to-date than Cohen's What Really Happened to the Dinosaurs (1977); in addition, it involves readers in the reasoning process and introduces them, with the greatest of ease, to some other scientific ideas of wider significance.