Hydrogen sulphide, H2S, is similar to water, except the central oxygen atom has been replaced by sulphur. The removal of the oxygen has eliminated the hydrogen bonding present in water, which means that H2S is a gas under normal atmospheric conditions. It is also extremely poisonous, much more so than hydrogen cyanide, but since it is incredibly foul smelling we can normally detect its presence long before the levels become toxic. (This is in contrast to HCN, which smells only slightly of almonds and can easily reach lethal levels before the victim is aware of it). Hydrogen sulphide has the characteristic stench of rotten eggs, and is formed when the sulphur-rich proteins in the egg albumen decompose. H2S is often formed when many sulphur containing proteins decompose, either after death or upon cooking. We also come across H2S as the particularly disagreeable odour in certain cases of flatulence, particularly after the perpetrator has eaten sulphur-rich foods, such as cabbage and sprouts!