This Unofficial Companion to Randall Munroe's book is not the actual Randall Munroe's book, What If?: Serious Scientific Answers to Absurd Hypothetical Questions. Do not purchase this book if you are looking for a full copy of Munroe's book. The Unofficial Companion includes an unofficial summary and critique of questions and scientific answers discussed in Munroe's book. It also includes additional questions and their scientific answers, helping readers with limited scientific background get a better grasp of some basics of science behind popular science books. (This book is not intended for readers with physics or engineering scientific background.) There are many real physical phenomena that have a certain degree of "wow factor" to them, and one does not have to destroy the Earth with a hair dryer, though, to make science interesting. This book discusses questions that were not addressed at all or were addressed only partially in Munroe's book: Munroe's book investigates what would happen if the Earth stopped spinning. The Unofficial Companion discusses how real winds and earthquakes affect the Earth's rotation. Munroe's book discusses how to make a mole planet. The Unofficial Companion discusses how gravity shapes planets and their atmospheres and how fight between gravity and dark energy affects the whole Universe. Munroe's book discusses how people can swim in a pool of spent nuclear fuel rods and briefly mentions that radioactivity heats up the pull (without explaining the nature of radioactivity). The Unofficial Companion discusses the nature of radioactivity and describes how radioactivity heats up our planet. Munroe's book discusses how a person can be harmed by a supernova. The Unofficial Companion discusses two different types of supernovae, how they can hurt our planet, and how hypernovae are more dangerous and far-reaching than supernovae. Munroe's book discusses how to send all the water away from the Earth. The Unofficial Companion discusses how the Earth's atmosphere actually slowly leaks into outer space. Munroe's book discusses time travel. The Unofficial Companion briefly discusses some ideas behind the quest to understand the nature of time. Munroe's book calculates the power generated by a sci-fi movie's character. The Unofficial Companion explains how you can measure your own horsepower while doing work against the Earth's gravity. Munroe's book considers sending a submarine into the orbit. The Unofficial Companion reminds readers that the Earth is located within the reaches of the Sun's upper atmosphere, the solar corona that streams outward in the form of the solar wind. The Earth's magnetic field prevents electrically charged particles of the solar wind from entering the vicinity of the Earth and reaching the Earth's atmosphere. The Unofficial Companion further discusses what would happen if the submarine were sent to travel through the solar corona. Munroe's book discusses the effects of a hypothetical situation when we all simultaneously use laser pointers to send intense, focused beams of visible light radiation toward the Moon. Considering that the Sun continuously sends visible light radiation toward the Earth and other planets of the Solar System, the Unofficial Companion tells the readers what would happen if all the Sun's radiation were sent toward the Earth. It also briefly discusses how the solar radiation changes with time. Munroe's book sends a person moving vertically upward away from the Earth; the Unofficial Companion discusses how atmospheric pressure changes with altitude and how high one needs move up above the Earth in order to become an astronaut. Munroe's book discusses flying airplanes on other planets. The Unofficial Companion explains how airplanes fly, how space rockets fly and how artificial satellites stay in their orbits.