solids and liquids are both forms of condensed matter; both are composed of atoms in close proximity to each other. but their properties are, of course, enormously different. while a solid material has both a well-defined volume and a well-defined shape, a liquid has a well-defined volume but a shape that depends on the shape of the container. stated differently, a solid exhibits resistance to shear stress while a liquid does not. externally applied forces can twist or bend or distort a solid’s shape, but (provided the forces have not exceeded the solid’s elastic limit) it “springs back” to its original shape when the forces are removed. a liquid flows under the action of an external force; it does not hold its shape. these macroscopic characteristics constitute the essential distinctions: a liquid flows, lacks a definite shape (though its volume is definite), and cannot withstand a shear stress; a solid does not flow, has a definite shape, and exhibits elastic stiffness against shear stress.