Life as we know it depends on the diverse properties of matter: solids and fluids. Just think about water in its various forms—liquid, solid (ice), and water vapor in our atmosphere, which eventually returns as rain or snow. And as a whole, the atmosphere contains a mixture of gases in an ideal combination to sustain life. Now think about the house you live in or the building you’re in at this moment. Buildings are intricate mazes full of a variety of solid materials that serve different functions. Solids and fluids also carry waves, especially sound waves, as well as electric current. In this chapter we’ll address some of the vital questions about how the properties of matter affect our lives.