When a body is supporting a load, a force is acting on it. This force will produce a change, perhaps not very noticeable, in the form of the body. Unless this load is so great as to cause a break or fracture, the elasticity, which has previously been defined as the tendency of the particles of a body to unite, or return to their original positions, will support the load. The forces of the body resisting the pull or pressure of the load are called stresses. The change of shape of the body producing these stresses is called a strain. To illustrate: The molecules of a piece of iron are held together by the force of cohesion, which is stronger in iron than in some other bodies. This force must be overcome in order to change the condition, form, or size of the iron bar, or to break it into parts. When the iron bar is supporting a load, the resistance which the bar offers to the pressure or pull of the load that tends to overcome the force of cohesion is called a stress. If the load is not very great, the particles of iron may be separated while the iron is supporting the load, but they will return to their original position as soon as the load is removed.

The elasticity of different substances varies. The degree of elasticity of the various materials is found by measuring the forces required to produce equal changes in four pieces of the same material of like dimensions. In case the load is very great and the particles of iron are separated to such an extent as not to return to their original positions when the load is removed, the structure of the iron is more or less broken down. This is very clearly shown by the change in appearance of polished surfaces of a metal in a stressed condition. The bright surface suddenly becomes dull when the stress exceeds the amount which affects the permanent structure. Another example of stress is seen when a large casting is lifted by a crane or derrick. The chains supporting the casting are then said to be "in stress" or "stressed."