Legend has it that, back in the fifth century A.D., St. Patrick exterminated Ireland's snakes by driving them into the sea. He would appear to have done a thorough job, because Ireland is free of native snakes to this day.

Except, Ireland never actually had snakes. Everything you've been told about St. Patrick's Day is a lie. (Ok, probably not all of it.)

So, if snakes can be found almost everywhere else the world, from Australia to the Arctic Circle, what makes Ireland so special?

For one, it's an island. The Irish Sea is 50-plus miles wide. That would be a long swim for a land animal. A sea snake might have an easier time of it, but sea snakes live in warm tropical waters, not the frigid Atlantic.

But, you may be thinking, the U.K. has snakes, and it's an island. That's true. But for a long time, neither Britain nor Ireland was home to snakes. The Ice Age made the islands inhospitable to reptiles, whose cold-blooded bodies need heat from the surroundings to function. The glaciers retreated around 10,000 years ago, exposing a land bridge between Europe and Britain, and another between Britain and Ireland, allowing easy passage to the islands.