Sulfur is a yellow solid that is a nonmetal. It is fragile (easily broken) and crystalline. It burns easily, releasing toxic fumes of sulfur dioxide. It has a very faint odor. If it is melted and cooled very quickly, it makes a rubbery form of sulfur that is called "plastic sulfur". It gradually turns back into the yellow brittle form. It does not dissolve in water. The smell normally known as "sulfur" comes from hydrogen sulfide and similar chemicals. These sulfides are produced when things decay without air.