Most matter on Earth exists in three classical states (or phases): solid, liquid, or gas. When water is in a solid state, such as ice, the water molecules are in fixed positions, and the water maintains a fixed volume and shape.
When water is in a liquid state, such as in a glass of water, the water molecules are close together but not in fixed positions, and the water maintains a fixed volume but will adapt to the shape of its container.
When water is in a gas state, such as steam or water vapor, the water molecules are relatively spread out, and the water will expand to occupy whatever volume is available.
There are also many non-classical states of matter. Two of the most common are the glass (or amorphous solid) state and the plastic crystal state. In the glass state, the molecules are in fixed positions, but those fixed positions are not in a crystalline structure or repeating pattern. In the plastic crystal state, the molecules are in fixed positions, but the molecules are free to rotate and maintain random orientations. Less common on Earth, but very common throughout the universe, is the plasma state. At extremely high temperatures, matter can enter a plasma state where electrons are separated from atoms, forming a cloud of high-energy ions and electrons. Plasma is formed in stars and as lightning passes through the air.
Understanding matter means being able to visualize and have a clear understanding of how the molecules that make up matter behave in their many different states. The more we can think of those molecules as concrete objects that we can analyze, the more effectively we will be able to study and predict the properties of matter.