Here's why:

* Elements are the fundamental building blocks of matter. They cannot be broken down into simpler substances by chemical means. Examples include:

* Carbon (C)

* Oxygen (O)

* Gold (Au)

* Hydrogen (H)

* Compounds are formed when two or more elements chemically combine in a fixed ratio. They can be broken down into their constituent elements through chemical reactions. Examples include:

* Water (H₂O)

* Carbon dioxide (CO₂)

* Sodium chloride (NaCl)

* Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means. Examples include:

* Sand and water

* Air (a mixture of gases)

* Salt and pepper

So, while compounds and mixtures can be considered "simpler" than complex molecules or materials, they are not the simplest form of a substance. That honor belongs to the elements.