Pink Floyd's rock and roll masterpiece, The Dark Side of the Moon, turned 40 years old this weekend. If you don't already own a copy or three, consider checking it out. It's the best album by one of history's best bands, and you can find as easily in any record store in Phuket as you can in Philadelphia.

It's in music sellers' best interest to keep a few copies in stock, you see: in four decades, Dark Side has spent more than 800 weeks on the Billboard 200. That's more than 15 years!

No surprise, really: if you're looking for a rock record to put on while you blaze a j, go for a drive or make passionate love, it doesn't get much better than Dark Side. Each year, a new crop of 15-year-olds seems to discover it.

Despite being beloved for at least three generations now, though, there are still a few mysteries that the album reveals only to the most dedicated Floyd freaks. Here are ten of our favorites: