Everything around you is made of matter, and matter comes in three main forms: solids, liquids, and gases. Solids keep their own shape, like a rock or a book. Liquids flow and take the shape of their container, like water in a cup. Gases spread out to fill whatever space they are in, like the air in a balloon. You can sort almost anything you see into one of these three groups by watching how it behaves.
Sort everyday objects into three labeled bins — solid, liquid, gas. Use sealed bags of ice, water, and air so students can squeeze, shake, and compare. Follow up with a class walk to find examples of each state in the school building.
Look around the room right now. The desk, the chair, the pencil — those are all solids. The water in your bottle is a liquid. The air you are breathing is a gas. These are the three main states of matter, and every piece of stuff you will ever encounter fits into one of them. Learning to tell them apart is the first step to understanding how matter works.
Solids are the easiest to recognize. A solid keeps its own shape no matter where you put it. A wooden block is the same shape on the table, on the floor, or in your hand. Solids also take up a definite amount of space — you cannot squeeze a rock into a smaller rock just by pressing on it. The tiny particles inside a solid are packed tightly together and locked in place, which is why solids feel firm.
Liquids are different. Pour water from a glass into a bowl, and the water changes shape to fit the bowl. Liquids flow because their particles can slide past one another, but those particles still stay close together. That is why a liquid takes up a definite amount of space even though it does not have a definite shape — a cup of water is still a cup of water whether it is in a tall glass or a wide dish.
Gases are the trickiest because you usually cannot see them. Air is all around you, but it is invisible. Gases spread out to fill whatever container they are in. Blow air into a balloon and the air fills the whole balloon. Open the balloon and the air escapes into the room, spreading out everywhere. Unlike solids and liquids, gases can be squeezed into a much smaller space — that is why you can pump air into a bike tire.
Here is a simple test you can always use: Does it keep its own shape? If yes, it is a solid. Does it flow and take the shape of its container but stay the same amount? That is a liquid. Does it spread out to fill all the space it can? That is a gas. Once you learn these three rules, you can classify anything.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.