Sulfur: The Key to Strong Hair, Feathers & Skin

The element that is most important for adding strength to hair, feathers, and outer skin is sulfur.

Specifically, sulfur is a key component of keratin, which is a fibrous structural protein. Keratin is the main protein that makes up hair, feathers, and the outer layer of skin (epidermis). Sulfur forms strong disulfide bonds between keratin molecules, cross-linking them and providing rigidity and strength.

Therefore, the answer is sulfur.