Particle theory
Particle theory tells us that everything around us is made up of tiny tiny pieces called particles. You can't see these particles because they're super small. These particles are always moving, but how they move depends on what they are part of.
In solids (like ice or a rock): The particles are packed tightly together and just wiggle in one place. That’s why solids keep their shape and don’t flow.
In liquids (like water): The particles are close but can slide past each other, so liquids take the shape of the container but keep the same amount of space.
In gases (like the air): The particles move around freely and spread out to fill any space. That's why gases don't have a fixed shape or size.
When you heat things, the particles move faster and can change from solid to liquid to gas. When it gets colder, they slow down and can change back.