Gases spread out to fill whatever space they are in, have no fixed shape, and can be squeezed into a smaller space. Air is the gas you interact with most — it fills your lungs, inflates tires, and pushes on everything around you. Even though you cannot usually see gases, they are real matter that has weight and takes up space. You can prove gases exist by trapping them and watching what they do.
Have students push an upside-down empty cup straight down into a bowl of water — the air trapped inside keeps the water out, proving the gas takes up space. Use syringes (without needles) to let students compress air and feel the push-back. Compare this to trying to compress water in a syringe, which barely budges.
Gases might be the hardest state of matter to believe in because you usually cannot see them. But air is all around you right now, pressing on your skin, filling your lungs, and holding up airplanes. Gases are absolutely real — they are matter, just like solids and liquids. The difference is that the tiny particles in a gas are spread far apart and move very fast, which is why gases are invisible and fill up entire rooms.
The first key property of gases is that they spread out to fill their container. Blow up a balloon, and the air inside fills the whole balloon evenly — not just the bottom or one side. Open a jar of perfume in the corner of a room, and soon the whole room smells like it. That is because gas particles zoom around in every direction until they fill all the space available to them.
The second key property is that gases can be compressed. If you push air into a smaller space, the particles just crowd closer together. Think about a bicycle pump: you push the handle down and force air into the tire. The air squeezes into a tighter space, and the tire gets firm. Try the same thing with water and you will barely be able to push the plunger at all — liquids resist compression, but gases give in.
Gases also have weight, even though it does not feel like it. A basketball pumped full of air actually weighs slightly more than a flat basketball. Scientists have weighed air very carefully: a room-sized amount of air weighs about as much as you do. That weight pressing down on us is called air pressure, and it is what keeps straws working — when you suck on a straw, you remove some air, and the surrounding air pressure pushes the drink up.
So remember: just because you cannot see something does not mean it is not there. Gases are invisible, but they take up space, have weight, can be squeezed, and push on everything around them. Learning to think about the invisible world of gases is a big step in understanding how matter really works.