So what is a solid? Solids are usually hard because their molecules have been packed together. The closer your molecules are, the harder you are. Solids also can hold their own shape. A rock will always look like a rock unless something happens to it. The same goes for a diamond. Even when you grind up a solid into a powder, you will see tiny little pieces of that solid under a microscope. Liquids will move and fill up any container. Solids keep their shape.
In the same way that a solid holds its shape, the atoms inside of a solid are not allowed to move around too much. This is one of the physical characteristics of solids. Atoms and molecules in liquids and gases are bouncing and floating around, free to move where they want. The molecules in a solid are stuck in place. The atoms still spin and the electrons will still fly around, but the entire atom will not change position.
Solids can be made up of many things. They can have pure elements or a variety of compounds inside. When you get more than one type of compound in a solid it is called a mixture. Most rocks are mixtures of many different compounds. Concrete is a good example of a man-made mixture.
The opposite of a mixture is something called a crystal. When a solid is made up of a pure substance and forms slowly, it can become a crystal. Not all pure substances form crystals because it is a delicate process. The atoms are arranged in a regular repeating pattern called a crystal lattice. A crystal lattice is a very exact organization of atoms.
A good example is carbon. Carbon actually makes more than one kind of crystal lattice, although most elements only make one. A diamond is a perfect crystal lattice while the graphite arrangement is formed into microscopic sheets.