Matter comprises all substances that take up space and have mass, and it may exist as a solid, liquid, gas or plasma. Light and energy are not matter because they have no mass and do not occupy space.
The molecules that make up a solid lie close together and are bound strongly to one another. Because of this, solids have a distinct shape of their own that is not dependent on a container. The molecular bonds of a liquid are weaker than those of a solid, meaning that liquids do not have fixed shape. A liquid takes the shape of its container, but it does not change in volume. Molecules in a gas are very weakly bound. Like liquids, gases take the shape of their container. However, gases also change volume, expanding to fill their entire container.
The fourth phase of matter is plasma. Plasmas exist only at the high temperatures and pressures of the sun or bolts of lightning. Plasmas occur as electrons tear away from atoms, leaving behind positive ions and free electrons. Most scientists consider plasmas to be fluids like liquids and gases. Substance are able to transition between the stages of matter given the application or removal of heat.