You may not realize it, but scientists are actually discovering new states of matter all the time. In recent years we’ve added quite a lot to the solid-liquid-gas list, from Fermion Condensates to Quark-Gluon Plasmas to Super-Matter!
First, there are five or six different phases or states of matter. Solid, liquid, gas, plasma, and two condensates. But let’s start with the basics. The atoms in solids are hooked up and ordered into a rigid structure- think steel, rocks, ice, crystals, Theoretically, anything can be a solid, even hydrogen. In 2016, two scientists published a paper claiming that they were able to put hydrogen under sufficient pressure to turn it into a solid- a shiny, metallic solid, no less.
If you add more energy to solids, you end up with liquids. Liquids, compared to solids, usually have a messier structure. They conform with their container, have a constant volume, and are usually non-compressible. But even liquids are still not completely understood and we’re learning more about fluid dynamics all the time. Add more energy to liquids, and we get gas. Gases have no order to their particles, no definite shape, and no definite size.