Tiny moving particles

Particles are very, very small. One drop of water holds more particles than anyone could count, and no ordinary microscope lets us see them. Because we cannot see a gas, how do we know it is there? A balloon helps. When you blow into it, it grows. Something invisible is filling it, so the gas takes up space. If you squeeze the balloon, you feel it push back. Scientists use models to think about what is too small to see. A model is useful, but every model has limits. Our dots show the arrangement, yet real particles do not look like flat coloured circles and are never still. A good scientist says both what the model shows and what it leaves out.