The solid state and the structure of Solids We all live on terra firma, the 29 percent of our planet’s solid crust that lies above sea level. And almost everything we do is tied to solids, living in houses, creating and marketing solid goods, eating solid foods, and so on. But if you are asked to define a solid, it might be difficult. A solid is one of those familiar things that are hard to put into words. A good definition of a solid is that it tends to keep its shape when it is left alone. But