Nootropics, popularly referred to as "smart drugs", are substances which boost human cognitive abilities (the functions and capacities of the brain). The word nootropic is derived from the Greek words noos or mind and tropein meaning to ward. Typically, nootropics are alleged to work by increasing the brain's supply of neurochemicals (neurotransmitters, enzymes, and hormones), by improving the brain's oxygen supply, or by stimulating nerve growth.