When I was a kid, I learned that there were three states of matter: solid, liquid, and gas. And then a little bit later, my science teachers tossed around the word "plasma" as another mysterious state of matter no one knew about. So I started labeling things -- water is a liquid, helium is a gas, oobleck is a... liquid? Wait, what's light, then?

It's easy to categorize most of what we see as solid, liquid, or gas. But when we start considering "things" such as light or fire, we realize there's a disconnect. Not every "thing" is in a state of matter.

What is a "thing"?

The first question we need to ask is what a "thing" is. Most of what we see is matter. Britannica defines matter as a "material substance"[1]. That doesn't really give us much information. Wikipedia says matter is "any substance that has mass and takes up space"[2]. Again, not much info, but maybe we can look at mass and volume, which are two defining qualities of matter.

Mass is surprisingly hard to define and has several different physics related definitions. One of them involves an object's resistance to being moved, another is related to gravity, another is related to quantum mechanics, etc. But these seem to be getting into a never ending spiral of definitions.

Going back to matter, then. Instead of trying to define the word itself, we can look to a specific form of matter -- an element. A chemical element is a "thing" (scientists like to say "substance") that can't be broken down into smaller, different things. The smallest unit of an element is an atom.

So maybe we can say that most "things" we encounter are made up of elements, which are then made up of atoms.

What about combinations of "things"?

The first question you should have is: what about things that aren't just one element? Almost anything you see in your daily life is made up of more than one element. In fact, a lot of elements are very dangerous and explosive when they get exposed to the outside world.

Most "things" we see are combinations of elements. Some "things" are chemical combinations, known as compounds. Others are physical combinations, known as mixtures.

What about non-"things"?

1. https://www.britannica.com/science/matter
2. https://en.wikipedia.org/wiki/Matter