While often categorized as vegetables in culinary contexts, mushrooms are actually the fruiting bodies of fungi. Genetically, fungi are more closely related to animals than to plants. Unlike plants, they do not perform photosynthesis and must 'eat' organic matter. Their cell walls are made of chitin, the same substance found in the shells of crabs and insects, whereas plant cell walls are made of cellulose. Mushrooms are a unique 'everyday food' because they are one of the few non-animal sources of Vitamin D; when exposed to UV light, they convert ergosterol into Vitamin D2. The common white button mushroom, cremini, and portobello are actually all the same species, *Agaricus bisporus*, just harvested at different stages of maturity.