Here's why:

* Proteins are polymers: They are long chains of smaller units called monomers.

* Amino acids are the monomers of proteins: Each amino acid has a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable side chain (R group).

* The R group determines the amino acid's properties: There are 20 different amino acids, each with a unique R group that gives it specific chemical properties.

* Peptide bonds link amino acids: These bonds form between the carboxyl group of one amino acid and the amino group of another.

So, to form protein molecules, you need a supply of the 20 different amino acids. These amino acids can be obtained from your diet (essential amino acids) or synthesized by your body (non-essential amino acids).