A solid is scientifically defined as having structural rigidity and particles arranged with a relatively stable shape and a relatively stable volume. Unlike other states of matter, solids do not expand to take on the shapes of their containers, nor do they expand to fill entire areas of volume.
Of the four fundamental states of matter, solids are the state with particles that are packed most closely together. Solids can generally be characterized as either amorphous or crystalline solids. Amorphous solids are unique in that they do not exhibit crystal structures; instead, they have irregular bonding patterns and may appear soft or rubbery. Crystalline solids, on the contrary, are hard and brittle and include ionic, metallic, atomic, network atomic and molecular solids.