from the Introduction: Have you ever seen pictures of astronauts floating in space? Have you ever wondered why we do not float off the earth? When you push open a door, you are using a force. Forces are pushes or pulls. Forces make objects move. One force, called gravity, always pulls us toward earth. It keeps us from floating off into space. Even astronauts floating in a space shuttle are being pulled toward earth by gravity. They are floating because the spaceship is also being pulled toward earth. Gravity makes us and every object fall toward earth when dropped. We measure the force of gravity by weighing objects. You can find your weight by standing on a bathroom scale. Gravity pulls you down against the scale. Your weight is the force gravity exerts on you. Weight is measured in pounds (lb) or kilograms (kg). Pounds are divided into ounces (oz). Kilograms are divided into grams (g). The weights of small things, like medicines, are measured in ounces or grams. In this book, you will learn how to change one kind of measurement, like kilograms, into another such as pounds.