ROBERT EHRLICH is a physics professor at George Mason University. He is the author of The Cosmological Milkshake, What If You Could Unscramble an Egg?, and Why Toast Lands Jelly Side Down.

• Introduction 1Big, Blue, Spinning Ball The Earth 3What if you dug a hole through the Earth? 4What if you dropped a rock down a hole through the Earth? 6What if the Earth were square? 8What if you lived at the North Pole? 10What if the Earth had no magnetism? 12What if the Earth spun very fast? 14What if the Earth suddenly stopped spinning? 16What if the Earth were covered by water? 18It’s Raining Frogs Weather and Climate 21What if the Earth were hit by a big meteorite? 22What if a day lasted a year? 24What if there were no seasons? 26What if it rained cats and dogs (or fish and frogs)? 28What if people could control the weather? 30What if the climate got much hotter? 32What if the sky were always cloudy? 34What’s the Matter with Matter? Forces and Energy 36What if things fell up, not down? 38What if things could pass through one another? 40What if water didn’t evaporate? 42What if drops of water were really big? 44What if water could flow uphill? 46Are You Seeing Things? Light and Sound 49What if everything were the same color? 50What if the sky were black during the daytime? 52What if light didn’t travel in straight lines? 54What if light traveled very slowly? 56What if you could see sounds? 58What if you could see in the dark? 60Into the Wormhole Time 63What if all past time were squeezed into one year? 64What if you could travel into the future? 66What if you could travel into the past? 68Candle Clocks and Supercomputers Inventions 71What if cars didn’t need fuel? 72What if electric lights hadn’t been invented? 74What if clocks hadn’t been invented? 76What if computers could think like people? 78Smart Pigs and Pet Dinosaurs Plants and Animals 80What if the dinosaurs hadn’t died out? 82What if we brought the dinosaurs back? 84What of all plants and animals disappeared? 86The Amazing, Changing Me People and Animals 89What if some animals were smarter than people? 90What if you were size of an insect? 92What if you were the size of a giant? 94What if you had a dog’s sense of smell? 96What if you could fly like a bird? 98What if you saw the world upside down? 100What if you had three eyes? 102What if you looked exactly like everyone else? 104What if you always knew what other people were thinking? 106The Superstar and the Big Cheese The Sun and the Moon 109What if the Sun stopped shining? 110What if the Sun doesn’t rise tomorrow? 112What if the Earth didn’t go around the Sun in a circle? 114What if the Earth had two suns? 116What if the Moon fell down? 118What if the Moon were much closer to the Earth? 120What if there were no Moon? 122What if you moved to the Moon? 124Nonstop Flight to Neptune Visiting Other Planets 126What if you visited Mercury? 128What if you visited Venus? 130What if you visited Mars? 132What if you visited Jupiter? 134What if you visited Saturn? 136What if you visited Uranus or Neptune? 138What if you visited Pluto? 140What if you visited planets in another solar system? 142Beam Me Up The Stars and Beyond 145What if you traveled to the nearest star? 146What if you traveled to the center of our galaxy? 148What if you traveled to another galaxy? 150What if you went to the edge of the universe? 152What if there are intelligent aliens? 154What if tiny aliens landed on Earth? 156What if aliens lived inside the Earth? 158What if aliens who looked just like us landed on Earth? 160What if aliens who wanted to talk only dogs landed on Earth? 162Glossary 165Index 175