Here's why:

* Carbon (C) forms the backbone of the amino acid, creating a central carbon atom bonded to four different groups: a hydrogen atom, an amino group (NH2), a carboxyl group (COOH), and a side chain (R group).

* Nitrogen (N) is present in the amino group (NH2) which is a defining feature of amino acids.

While other elements like oxygen, hydrogen, and sulfur are also common in amino acids, carbon and nitrogen are the only ones present in *every* amino acid.