Here's the whole idea in one breath: everything is made of unimaginably tiny particles, always moving, and the three states are just those same particles arranged and moving differently. Not three different substances — the same particles, three ways.
Take an Ice Cube on a Hot Day
Watch a forgotten ice cube. It starts hard and holding its shape. Then it sags into a puddle. Leave the puddle in the sun and it vanishes into the air. Same water the whole time — it never became something else — yet it went solid, then liquid, then gas. Something is changing, but it isn't what the water is. It's how the bits of it are packed, and how fast they're jiggling.
The One Secret: Tiny Particles, Always Moving
Zoom in further than any microscope and matter isn't smooth — it's grainy, made of countless tiny particles. They're far too small to see, and they're never still. In a cold solid they only shudder on the spot. Warm them and they jiggle harder. Make them hot enough and they break loose from each other entirely and zoom around. Heat, really, is just particle movement: hotter means faster.
So the three states line up like this. In a solid the particles are packed tight in a fixed pattern, only vibrating — which is why a solid keeps its shape. In a liquid they're still touching but the pattern has broken, so they can slide and flow and take the shape of the cup. In a gas they've flown apart with big empty gaps between them, racing in every direction, filling whatever room they're in.
Drag the slider in the toy from cold to hot. Same particles, same number — you're only giving them more energy, and the state changes itself in front of you. That's the whole engine of this topic, and everything else is just learning to read it.