S – sulfur – in its pure form is a bright yellow, crystalline solid. When sulfur burns, it gives off a bright blue flame and can melt into a red liquid. Large accumulations of native sulfur are rare, normally found in places of volcanic activity. Nowadays, sulfur is produced from petroleum, natural gas, and related fossil resources. Sulfur forms a distinctive product if heated with hydrogen – a gas called hydrogen sulfide. This compound can be recognized by its very strong and unpleasant smell of rotten eggs. Nowadays, most sulfur is used to make sulfuric acid, but it is also used in the making of cement, rubber, pesticides, and traditionally in black gunpowder. Furthermore, it is one of the most abundant elements in the human body, comprising about 0.3 % as a component of proteins and enzymes. It also plays a key role in the formation of strong bones, healthy hair, and nails.