A solid is something which is stable in its shape. Although the three forms of matter (solid, liquid, and gas) all contain atoms, molecules, and ions, the particles of each behave very differently. Outside of taking up space, the properties of solid matter are as follows.
1. The particles in a solid are packed together very tightly and do not move about (in contrast to the particles of a gas or liquid).
2. The particles of a solid are rigid, meaning they are "fixed" in both volume and shape. It is very difficult to compress a solid given the packing of the particles.
3. The packing of the particles is formal (meaning the particles line up and are not sporadic).