Here's why:

* Amino Acids: Proteins are made up of amino acids, and some amino acids contain sulfur.

* Cysteine: The amino acid cysteine is the primary sulfur-containing amino acid found in proteins.

* Disulfide Bonds: Cysteine residues can form disulfide bonds, which play a crucial role in stabilizing protein structure.

* Other Roles of Sulfur: Sulfur is also important in other biological functions, like the metabolism of carbohydrates and the detoxification of harmful substances.

Let me know if you have any other questions!