When it comes to human reproduction, men have it easy. They enjoy all of the fun of procreation, but don't have to carry the baby or deliver it. When it comes to preventing pregnancy, however, the advantage lies clearly with the fairer sex. The one egg released every month by a woman makes an easy target for a variety of contraceptive interventions. As a result, women have numerous choices for contraception.

Men, on the other hand, present a greater challenge. Driven by eons of evolution involving complex processes of the endocrine and reproductive systems, men produce tens of millions of sperm per day. To prevent pregnancy, every one of those specialized cells -- think of them as DNA-packed Olympic swimmers -- must be blocked, hamstrung or killed. It's a daunting task, especially when you consider the timelines involved. It takes 75 days for sperm fashioned in the testes to mature in a tightly coiled companion structure known as the epididymis. That means an intervention that blocks sperm production won't become fully protective for two and a half months. And men who go off such an intervention would have to wait the same length of time before their fertility returns.

Because of these challenges, the search for the perfect male contraceptive -- one that's 100 percent effective, easy to use, reversible and free of side effects -- has been elusive. Condoms can fail; vasectomies are permanent and the male birth control pill hasn't materialized into a viable solution. Hope, however, may soon arrive in the form of a clear gel that's injected into the vas deferens, one of the body's main highways for sperm. The gel will be marketed in the U.S. under the name Vasalgel, but the technique is known as reversible inhibition of sperm under guidance, or RISUG. Don't bug your urologist just yet, though -- the gel still has miles to go before it receives approval from the U.S. Drug and Food Administration. Even in India, where a biomedical engineer invented the technique and has studied it for decades, RISUG still languished in phase III clinical trials as of spring 2012.

What exactly is RISUG, how does it work and how much does it cost? Is the gel safe in men who use it for years? And is it truly reversible, as its name suggests? We'll answer those questions on the following pages, but we'll begin -- in a bit of editorial foreplay -- with a story about water purification.