I was discussing important matters recently with a friend who happens to have a Ph.D in pharmacology, and she told me something that really rattled my molecules: The Olsen twins, Mary-Kate and Ashley, aren't actually identical--no matter how much they resemble each other on TV.

Like, oh no, I thought. How could it be? This revelation of the perky twosome's nonidentical nature is almost as hard for me to believe as the vastness of the teen actresses' billion-dollar fashion and entertainment empire.

I felt a burning curiosity to know whether the statement was true, and yet I could not bring myself to track down their mother and ask. So naturally, I searched the Internet instead.

My search generated a host of results. I poked around a bit, and though I did locate information about the "mary-kateandashley brand" of jeans, I couldn't find much information about the kind of genes I was seeking.

And while I did find some sites that reported that the twins are fraternal, these sites didn't back that up with anything beyond saying that one is left-handed and one is right-handed.

As I have since learned from Nancy L. Segal, a professor of psychology at California State University, Fullerton, this doesn't prove a thing. Twins can be identical and yet use opposite hands for writing. Blood tests can generally tell you for sure whether twins are identical, though sometimes you need to look at DNA markers to ferret out the truth.

To really know whether Mary-Kate and Ashley are fraternal or identical, we'd need a little of their blood. But alas, that's one of the few things they're not selling on their Web site.

Fortunately, by this time I was finding myself more interested in the mysteries of twins in general than in the details of this specific set.

Did you know that every August twins from around the globe descend upon Twinsburg, Ohio, for the annual Twins Days Festival. In the past, participants have included twin nuns named Celeste and Celestine, as well as Craig and Mark Sanders and their wives, Darlene and Diane. Craig and Mark are twins, as are Darlene and Diane. They met at the festival four years ago, married, and now live next door to each other in Houston, Texas. Craig and Diane are now the parents of twin boys. If it weren't for the fact that Mark and Darlene have a single daughter, their neighbors would probably think they were seeing double.

But I digress. Why are twins so fascinating, anyway?