In describing the mechanical properties of materials, one is interested in understanding the response of the materials to force. For example, consider the forces that are exerted on materials as we walk around. The forces arise because our bodies are being acted on by gravity and this force is acting on each particle of our body. A force that acts on every particle of a body, animate or not, is known as a body force. The force produced by our bodies is then transmitted to the floor through our feet. As the force is being transmitted via a surface, it is known as a surface force. Now, let us consider what is happening to the floor as we transmit this force. In general, we do not notice much of a reaction but, following Newton's Third Law, we know for every action there is an equal and opposite reaction. In a way this is rather remarkable, as it indicates the floor is pressing back on our feet with exactly the same force as that caused by our weight. If the reactive force was less, we would sink and if it was too high, we would rise. To understand the mechanical properties of materials, it is important to understand how this reaction arises and as materials scientists we are interested in determining whether this reaction can be controlled.