Here's why:

* Organic compounds are defined as molecules that contain carbon.

* The 4 main classes of organic compounds are:

* Carbohydrates: Composed of carbon, hydrogen, and oxygen.

* Lipids (fats and oils): Also composed of carbon, hydrogen, and oxygen, but with a higher proportion of hydrogen.

* Proteins: Composed of carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain sulfur.

* Nucleic Acids (DNA and RNA): Composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

While other elements are present in these classes, carbon is the defining element that makes them "organic".