Early on, everyone learns that there are three basic states of matter: solid, liquid, and gas. That's not entirely correct, as there are also fun things like plasma (the fourth state of matter) and not-quite-states like supercritical fluids, but it's correct and reasonable enough for us to go on. Also, usually, we think of these states as existing in a line: solid, at a high enough temperature, becomes a liquid; liquid, at a high enough temperature, transitions to a gas. We imagine these states to exist on a temperature line like so: