There are three main ways to classify matter: elements, compounds, and mixtures. Elements are substances that cannot be broken down further, examples include anything on the periodic table. Compounds are substances made of two or more elements bonded together in a fixed ratio, such as NaCl or CO2. Mixtures are combinations of elements or compounds that can be combined in any ratio, and can be either homogeneous, where the composition is uniform throughout, or heterogeneous, where the composition varies. Common examples of each are provided.