As the hippies used to remark, it's a gas.

"It," in this case, is nitric oxide, often called by the forbidding acronym NO, which first gained public acclaim when its key role in erectile function was figured out - leading boatloads of aging flower children (and others) to say "yes" to Viagra, the potency blockbuster generically known as sildenafil citrate that works by inhibiting NO's breakdown in the body.