The state of matter—whether solid, liquid, or gas—depends on the strength of the forces between particles relative to the substance's thermal energy. In the solid phase, there are relatively strong intermolecular forces, and the particles stay in the same positions relative to one another, just vibrating back and forth. This gives the solid state a definite shape and definite volume. In the liquid phase, some of the intermolecular forces have been overcome by thermal energy, so that particles are able to move with respect to one another. However, there are still enough interactions between particles to keep them close to one another. Therefore, liquids have definite volumes but indefinite shapes. And for a gas, the intermolecular forces have been completely overcome by thermal energy, so that the gas particles are very far away from each other. This gives gases a low density, indefinite shape, and indefinite volume. In this way, we can explain some of the properties of matter by considering the forces between particles relative to thermal energy. Solids can be characterized as crystalline or amorphous. Crystalline solids have a regular ordered structure; some examples of crystalline solids are salts, ice, or sugar. Amorphous solids have no long-range order. Some examples of amorphous solids would be glass, wax, or most plastics. We can change between states of matter by changing temperature, pressure, or both. For example, increasing temperature will transition most substances from solid to liquid to gas.