Facts change all the time. The age at which women should get a mammogram has increased. Smoking has gone from doctor recommended to deadly while the healthiness of carbs and fat seems to be in constant flux. We used to think the Earth was the center of the universe, that Pluto was a planet, and that the brontosaurus was a real dinosaur. What we know about the world is constantly changing. Samuel Arbesman is an expert in scientometrics, literally the science of science-how we know what we know. It turns out that knowledge in most fields evolves in systematic and predictable ways, and understanding that evolution can be enormously powerful. For instance, knowing how different branches of medicine overturn their bodies of knowledge can improve the way we train (and retrain) physicians. The Half-Life of Factsfeatures fascinating examples from fields as diverse as technology and literature. It will help us find new ways to measure the world while accepting the limits of how much we can know with certainty.