Viagra is one of the best-known drugs of all time. Nearly every adult in America has heard of the drug and can tell you what it does.

In the years since it was introduced in 1998, former Republican presidential nominee Bob Dole has served as a spokesman for the drug, manufacture of counterfeit pills has gone through the roof, and Viagra jokes are now a permanent feature of the pop culture landscape.

What's the big deal about "the little blue pill"?

It's simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection.

How does Viagra do that? And why does Viagra work only if the man is sexually stimulated? For that matter, what causes an erection in the first place? In this article, we'll answer all of those questions and more.

This is a fascinating story -- it involves the technology of the human body and the techniques that scientists use to control its different parts with drugs. And in the case of Viagra, the story starts with the penis.