The structure of matter
In nature matter appears in three states: solid, liquid and gas. A solid keeps its own shape and volume (wood, a pencil, a desk). A liquid keeps its volume but takes the shape of its container (juice in a glass or a bowl). A gas keeps neither shape nor volume and fills the whole container (the air in a room). All matter is made of tiny particles that we cannot see; a molecule of a substance is a very small particle of that substance, for example a water molecule. The particles do not change when the state changes: ice, water and water vapour contain the same particles. The difference is how the particles are arranged, how much they move and how they act on one another.
Experiment: take three plastic syringes without needles, one with air, one with water and one with dry sand. Close the tip with a finger and push the plunger. The air is clearly compressed, the water hardly at all and the sand not at all. Record the result in a table. Safety: use only needle-free syringes, do not spill water (slipping risk) and keep sand away from eyes.