What is the difference between psilocybin and psilocin?

Psilocybin and psilocin are chemical counterparts. The former is a psychedelic prodrug found in hundreds of varieties of Psilocybe fungi, while the latter occurs when psilocybin is broken down inside the human body. You cannot have one without the other, but it is the psilocin that binds with serotonin receptors in brain cells to produce mind-altering effects.