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

Here's why:

* Elements are the basic building blocks of matter. They are made up of only one type of atom.

* Compounds are formed when two or more different elements chemically combine. These can be broken down into their constituent elements through chemical reactions.

Examples of elements:

* Oxygen (O)

* Gold (Au)

* Carbon (C)

* Hydrogen (H)

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