While there are a few exceptions, the presence of carbon is the defining characteristic of organic molecules.

Here's why:

* Carbon's Bonding Ability: Carbon has the unique ability to form four bonds with other atoms, including other carbon atoms. This allows for the creation of long chains, rings, and complex structures, leading to the vast diversity of organic compounds.

* Diversity of Functional Groups: Carbon's bonding capacity also enables the attachment of various functional groups (like hydroxyl, amine, carboxyl) to organic molecules. These functional groups give organic molecules their distinct properties and reactivity.

While carbon is the defining element, many organic molecules also contain other elements like hydrogen, oxygen, nitrogen, and sulfur.