Solid Basics
So, what is a solid? Solids are usually hard, because their molecules have been packed together. You might ask, "Is baby power a solid? It's soft and powdery." Baby power is also a solid. It's just a ground down piece of talc. Solids can be hard, soft, big or small like grains of sand. The key is that the solids hold their shape and they don't flow like a liquid. 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 pieces of that solid under a microscope. Liquids will flow and fill up any shape of container. Solids like 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 thephysical 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 a specific structure or arrangement of atoms. The atoms still spin and theelectrons 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. Granite is a mixture you might find when you are hiking around a park. Granite is made up of little pieces of quartz, mica, and other particles. Because all of the little pieces are spread through the rock in an uneven way, scientists call it a heterogeneous mixture. This is important because there are different concentrations of specific particles in different parts of the rock. In one place there might be a lot of quartz and very little feldspar, but only a few inches away those amounts might flip.
Crystals
On the other end of the spectrum from 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 very delicate process. The atoms are arranged in a regular repeating pattern called acrystal lattice. A crystal lattice is a very exact organization of atoms. A good example is carbon (C). A diamond is a perfect crystal lattice of carbon, while the graphite arrangement of carbon atoms is a more random and disorganized. You can find graphite in your pencils. For carbon, those two different structures (crystal lattice vs. random arrangement) are called allotropes.
