In a gas the individual molecules move freely through space,
and do not touch except at the moments of collision, from which
they rebound. A gas has neither a fixed shape nor a fixed volume;
it adapts to the shape of its container and can be expanded or
The molecules of
a liquid are in contact with one another, but have enough energy
to slip past one another and change their positions. Therefore,
a liquid has a relatively fixed volume, but no definite shape.
In a crystalline solid the molecules are packed against one another
in a regular pattern, and do not have enough energy to break that
pattern and slide from one place to another. Crystals have a definite
volume and shape, and work must be done to deform or break them.