Olfactory receptors are specific to one type of odorant molecule. The combination of receptors which are activated tell your brain what you are smelling. For example, if one certain smell receptor binds hexanoic acid (a 6 carbon chain with a carboxylic acid attached) only one receptor recognizes it and you will smell sour and goat-like. But if your receptors bind nonanoic acid (a 9-carbon chain with a carboxylic acid attached) about eight different receptors will recognize it, and you will smell cheese and nuts.