Solid - atoms are tightly packed, and don't move around (desk, pencil, backpack)
Liquid - spaces between atoms that move slightly all the time (water, soda, juice)
Gas - atoms are always moving. There is so much space between the atoms that you can move around in it. (air, steam)
Adding energy, in the form of heat, can change a solid to a liquid. And if we continue to add heat, our liquid will turn into gas.
Removing energy from a gas can create liquid. And if we continue to remove heat, our liquid will freeze into a solid.
The Big Question!
What's the matter with a bowl of chicken noodle soup? (what states of matter can you find in a bowl of chicken noodle soup?)