Matter most commonly exists as a solid, liquid, or gas; these
states are known as the three common phases of matter. Solids
have a de nite shape and a speci c volume, liquids have a de nite
volume but their shape changes depending on the container in
which they are held, and gases have neither a de nite shape nor
a speci c volume as their molecules move to ll the container in
which they are held. Liquids and gases are considered to be uids
because they yield to shearing forces, whereas solids resist them.
Note that the extent to which uids yield to shearing forces (and
hence ow easily and quickly) depends on a quantity called the
viscosity. Liquids deform easily when stressed and do not spring
back to their original shape once the force is removed because the
atoms are free to slide about and change neighbors that is, they
ow (so they are a type of uid) with the molecules held together
by their mutual attraction. Because the atoms are closely packed,
liquids, like solids, resist compression
