Do you ever wonder what things are made of? The glasses you drink from, the clothes you wear, the floor beneath your feet?
You may think, well that´s easy! Glass, fabric, and wood. But more than that; what´s the wood made of? Trees? What are trees made of?
What about on a smaller level?
Take a glass of water for example, it´s actually made of tiny things called molecules.
In liquids, the molecules stay in a certain place, but have enough energy to slide past each other.
Depending on whether you add or take away energy, matter (anything that has mass and takes up space) can change its state. Imagine freezing or boiling water; when you freeze water, it changes from a liquid to a solid. When you boil water, it changes from a liquid to a gas.
If you were to change a liquid to a solid, you would need to take away energy. The molecules wouldn't be able to move past each other, they would instead stay in a rigid pattern and shake in place.
And if you were to change a liquid to a gas, you would add even more energy. The molecules in gas don't stay in place at all, they move around as freely as they want.