A substance that cannot be broken down into other substances is called an element.

Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom.

* Compounds are formed when two or more different elements combine chemically in a fixed ratio.

* Mixtures are combinations of substances that are not chemically bonded. They can be separated by physical means.

Examples of elements include:

* Oxygen (O)

* Hydrogen (H)

* Gold (Au)

* Carbon (C)

* Iron (Fe)

Let me know if you'd like to know more about elements!