We learned that not only the things we see exist, because invisible gases like carbon dioxide are real and can change what happens around us. Matter can be a solid, liquid, or gas, and it can change from one state to another with heat or cold. Density means how much matter is packed into a space, and it was very important in the experiment because CO₂ is denser than air, so it sank to the bottom of the beaker and pushed out the oxygen, making the candle go out.