Gravity is one of the most important forces in the universe - invisible, but vital to our existence. But how does it work? A True Book: Physical Science allows readers to find answers to puzzling questions as well as discover how people have used physical science to make countless innovations throughout history. This series includes an age appropriate (grades 3-5) introduction to curriculum-relevant subjects and a robust resource section that encourages independent study. Readers will learn how gravity affects all matter, what effects changing gravity can have on human bodies, and how gravity interacts with other forces. They will also conduct gravity-related experiments to see this force in action.