Here's why:

* Mixture: A combination of two or more substances where each substance retains its individual properties. The substances can be separated by physical means (like filtration, evaporation, etc.).

* Pure substance: A substance composed of only one type of atom or molecule. It has a fixed composition and uniform properties throughout.

Examples of mixtures:

* Salt and pepper: You can easily see the individual grains of salt and pepper, and you can separate them using a sieve.

* Sand and water: The sand settles to the bottom, and you can pour the water off.

* Air: A mixture of gases like nitrogen, oxygen, carbon dioxide, and others.

Let me know if you'd like more examples!