Science: Asparagus Revisited (With Bonus Urine!)

In my previous recipe, I mentioned my love of asparagus. I was talking to a friend online as I cooked, and he told me that although he loved asparagus, he hated how it made his urine smell. I've heard people mention in the past that asparagus gives their urine a distinctive sulfurous stench, but I personally have always been unaffected. So, why does asparagus make your pee smell? And why does it happen to some people and not others? The answer, it turns out, is more interesting than you might think.