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 compressed.

LIQUID:

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.

SOLID:

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.