Strain inside a material may arise by various mechanisms, such as "stress" as applied by external forces to the bulk material (like gravity) or to its surface (like contact forces, external pressure, or friction). Any strain (deformation) of a solid material generates an internal "elastic stress", analogous to the reaction force of a spring, that tends to restore the material to its original non-deformed state. In liquids and gases, only deformations that change the volume generate persistent elastic stress. However, if the deformation changes gradually with time, even in fluids there will usually be some "viscous stress", opposing that change. Elastic and viscous stresses are usually combined under the name "mechanical stress".