While we treat them as vegetables in the kitchen, mushrooms are technically fungi. They are more closely related to animals than to plants in the tree of life. Unlike plants, mushrooms do not have chlorophyll and cannot produce their own food through photosynthesis. Instead, they release enzymes to decompose organic matter and absorb nutrients. This is why you often find them growing on decaying wood or soil. Their cell walls are made of chitin, the same material found in the exoskeletons of crabs and lobsters, whereas plants use cellulose. Mushrooms are also the only produce item that can produce Vitamin D; when exposed to sunlight (UV rays), they convert ergosterol into Vitamin D2, just as humans produce Vitamin D from the sun.