Read the following and answer the question.   
Everything found on Earth can be categorised into one of the three states of matter: solid, liquid or gas. In order to place a matter under any one of these categories, we need to study its properties, namely, shape, mass and volume. Mass is the amount of matter an object has. Volume is the amount of space the matter takes up. Solids are easy to identify. They have a definite shape, mass and volume. A piece of wood is solid. It is made up of tiny particles that are packed closely together. The particles give the solid a definite shape that does not change. Look around your classroom, you will find so many solids—desks, tables, chairs, blackboards, doors, windows, chalks, duster and books. Liquids do not have a definite shape, but they do have definite mass and volume. The particles that make up a liquid can move around a bit. Liquids can change shape by flowing. If you've ever spilled a glass of water on the table, then you know it spreads out across the table. It does this because water takes the shape of the table. Since liquids do not have a definite shape of their own, they will take the shape of the containers into which they are poured. Gases neither have a specific shape nor a definite volume. Like liquids, gases take the shape of their containers. The atoms in a gas are spaced farther apart than in a solid or a liquid, and being spread out like this allows them to move around freely. If gas is not in a container, it will spread out all around. Think about the air we breathe every day. The air is spread across the empty space all around us. 
According to the passage, solids are easy to identify because they: