Known to be behind the characteristic odor of rotting eggs, sulfur is essential for all living cells. Cells make proteins that form strong chemical bonds called disulfide bridges between two adjacent sulfur atoms. These bridges give strength to our hair, outer skin, and nails. Eggs are loaded with sulfur because disulfide bridges are needed to form feathers, which explains why eggs smell on rotting. Because sulfur is easy to smell, natural gas lines–which are normally odorless–have sulfur additives to help people identify and smell a gas leak when it occurs.