Answer
Solids, liquids and gases are differentiated by the spacing and movement of their molecules. Solids are densely packed, liquids more loosely packed, and gases are free moving.
Work Step by Step
In solids, closely arranged atoms or molecules are held tightly and may only vibrate. A liquid's atoms/molecules are also closely arranged, but less so and they are able to move around each other. This is what allows water to flow freely. In a gas, atoms/molecules are spaced entirely apart. Because of this they are compressible.
