Solids, liquids, gases, and plasmas: these words should be quite familiar to you because they are the four phases of matter, which are simply the different forms matter can take on. What's neat is that many substances can exist as more than one phase. Take water, for example: water can exist as a solid (ice), a liquid (liquid water), and a gas (water vapor).
The difference between these states is the amount of energy. Solids have the least amount of energy, which is part of why their particles hang so tightly together. Liquids have more energy than solids, which is why they will take on the shape of their container but only up to the surface.
Gases have even more energy than liquids. So much more in fact that their particles spread out to fill the entire space of their container. Gas particles have so much energy that they just can't keep still. They fly around in all directions, putting as much distance as possible between themselves and the rest of the gas particles.
Plasmas are ionized gases, and in their natural form are uncommon on Earth. You've seen them as man-made things, like neon signs and fluorescent light bulbs. But in the rest of the universe, plasma is actually the most common phase of matter! Most stars are plasma, as are the northern lights you see around the Polar Regions. Plasma only exists under certain conditions though, so we'll end our discussion of it here for this lesson.
