Here's why:

* Elements are the simplest pure substances that cannot be broken down further by chemical means (e.g., oxygen, carbon, hydrogen).

* Compounds are formed when two or more elements combine chemically in a fixed ratio (e.g., water (H₂O), carbon dioxide (CO₂)).

* Mixtures are combinations of two or more substances that are not chemically bonded and can be separated by physical means (e.g., salt and pepper).

Dogs are complex living beings made up of many different types of cells, tissues, and organs. They are composed of numerous elements, compounds, and mixtures, but they are not classified as any of those categories themselves.