Psilocybin

Psilocybin is a chemical found in the Psilocybe mexicana species of mushrooms. 
Psilocybe mexicana mushrooms are also called Magic Truffles 
The mushrooms (over 200 species of Basidiomycota) are typically found in tropical and subtropical areas of the United States, Mexico and South America. 
It is the ingredient that produces hallucinations and other experiences of an altered consciousness in people who consume the mushroom. Psilocybin is a crystalline tryptamine, a naturally occurring nitrogen compound, chemically similar to the human neurotransmitter DMT (dimethyltryptamine). Psilocybin is concentrated in specialized cells called sclerotia underneath the mushroom tops. Usually a person will simply eat the sclerotia or boil it in water to make a tea. The effects last about six hours, and may include hallucinations that gradually grow further apart until they disappear. Some people get silly under the influence of psilocybin, but others become more introspective. 
Each mushroom contains less than one half of one percent of psilocybin and trace amounts of psilocin, another hallucinogen. Both substances can be synthesized in a lab. 
Mushrooms containing psilocybin can be either fresh or dried and have long, thin stems topped by dark gilled caps. Like many hallucinogenic substances, mushrooms that contain psilocybin can be eaten, brewed in tea, coated with chocolate or added to other foods. Once inside the body, the digestive system will break down psilocybin to produce psilocyn.