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:

10. Paul McCartney ended up on the cutting-room floor. Dark Side of the Moon made brilliant use of voices throughout the album, and most of them came from folks who simply happened to be hanging around Abbey Road when it was recorded. Roadies, doormen, and even the members of Wings were posed questions that lyricist Roger Waters had written on cue cards, like "When was the last time you were violent and were you in the right?" and "What does the phrase 'The Dark Side of the Moon' mean to you?"

Paul McCartney was among those interviewed, but his voice didn't make the cut. Perhaps Sir Paul was being a tad too guarded or cheeky with his answers: Waters found he got better, weirder stuff from guys like the studio's Irish doorman, Gerry O'Driscoll, who let the band know that, matter of fact, it was all dark.

9. The album was nominated for a single Grammy... ...and not for the band. The only man honored for his work on the album with a Grammy nomination was its producer, Alan Parsons, who parlayed his newfound notoriety into his own successful recording group, the Alan Parsons Project. Never best-known for their humble attitudes toward creative credit, Floyd's members have downplayed Parsons' role in capturing Dark Side in the 40 years since, although they did ask him back for Wish You Were Here. (He declined.)

In truth, Parsons probably did deserve this Grammy. The engineer made use of some of the most cutting-edge studio techniques at the time to create the 16-track, quadrophonic mixes. Not bad at all, when you consider everything was being put together using tape and razor blades.

8. It's been 19 years since "Pink Floyd" performed the album in full. DSoTM was performed by the David Gilmour-led incarnation of Pink Floyd on select dates of the band's final tour in 1994. If it doesn't quite feel that long, it's probably because various members have played suites of music from the album live on tour several times since, including Roger Waters taking the entire album around the world with his solo band between 2006 and 2008. That tour hit the Woodlands on May 4, 2008.

7. Clare Torry was initially shocked the band liked her "Great Gig in the Sky" vocals. The original version of "The Great Gig In the Sky" that the band worked out live was essentially an organ solo featuring tape loops of Bible passages and religious speeches. For the album version, the lead was switched to piano, but the band couldn't find any samples they liked. They decided to try a singer.

Parsons found Clare Torry, a 22-year-old session vocalist. Unbeknownst to many who would later dry-hump to her voice, Torry was extremely white and extremely British. The band was largely at a loss on what instructions to give, so Torry imagined herself as an instrument and tried to interpret the song's theme of mortality.

After two full takes, the band dismissed her. She assumed they hated it and forgot about the session. While they played it cool, though, Pink Floyd was delighted with Torry's singing. She didn't realize they'd used her voice until she saw the album in a store and found her name in the credits.

6. It's not the only Floyd album acid freaks sync up with movies.

Ever since the mid-'90s, stoners, trippers and the extraordinarily bored have been blowing their own minds by syncing up Dark Side with MGM's classic The Wizard of Oz film and watching a few coincidental synchronicities occur, including the Scarecrow's brainless dance during "Brain Damage."

While it's cool to think about the band orchestrating a bizarre soundtrack to a golden Hollywood oldie, Waters, Gilmour and the others have bristled at the very suggestion that their opus had anything to do with the Emerald City.

The real mindfuck, of course, is that Floyd fans have actually tried syncing up other albums and films, including The Wall with Disney's Alice in Wonderland and Meddle with 2001: A Space Odyssey. Is it a stretch? Um, yeah. Lots of luck getting high enough for this sort of experimentation to be enjoyable in the slightest.

5. It was almost named Eclipse (A Piece for Assorted Lunatics). Despite Floyd's space-rock reputation at the time, the title Dark Side of the Moon was intended to be an allusion to a journey into madness, not outer space. The name was very nearly changed altogether, however, when the band discovered they'd been beaten to the punch.

British blues-rock duo Medicine Head release their own album titled Dark Side of the Moon in 1972 on John Peel's Dandelion label. Worried about being labeled as biters, the chagrined Floyds considered changing the name of their record to Eclipse (A Piece for Assorted Lunatics) for a time. But when Medicine Head flopped, they decided to move forward with the original name and hope no one noticed. (No one did.)

4. That insane laughter belongs to Naomi Watts' dad. The repeating, demented laughter heard on "Speak to Me" and "Brain Damage" belongs to Peter Watts, Pink Floyd's road manager at the time. He was also responsible for snippets of dialogue such as "I never said I was frightened of dying" from the beginning of "The Great Gig in the Sky." All of these bits were taken from the cue-card interview sessions orchestrated by Roger Waters.

Watts left Floyd in 1974 and sadly passed away only a couple of years later. Not before siring a daughter, however -- Naomi Watts, who go on to star in Hollywood films like King Kong and Mulholland Drive.

3. The rock and roll classic helped fund a comedy classic.

Like everybody in Great Britain who was worth a shit in the early 1970s, Pink Floyd were massive fans of Monty Python's Flying Circus. In fact, Alan Parsons would later say that the band's frequent breaks in recording to watch the show afforded him time and opportunity to experiment with different mixes and effects in the studio.

Floyd stepped up and showed their appreciation for the troupe of comedy innovators by using a portion of the jillions of dollars (er, pounds) earned by Dark Side of the Moon to help fund the production of Monty Python and the Holy Grail, one the most hilarious, daring and influential British comedy films of all time.

2. The album leaked nearly a year early. Pink Floyd developed the musical ideas that would become DSoTM on the road, playing early versions of the songs to audiences who, naturally, had no idea what they were listening to. By early 1972, the band was performing the full cycle, more or less. At their February 20 gig at London's Rainbow Theatre, somebody liked it enough to record a bootleg.

The band was horrified when the bootleg appeared in record stores a year before the album was to be released, certain that the popular boot would harm the actual album sales. Somehow, we have to think they were ultimately satisfied with DSoTM's sales.

1. Dark Side of the Moon was premiered in the London Planetarium. It seems an all-too-obvious place to hold the premiere, in retrospect. Dark Side of the Moon laser light shows have ruled late-night planetarium shows for decades now, including a memorable run at Houston's Burke Baker Planetarium. At the time, however, it was an incredibly novel idea to listen to a rock LP in a dome-ceilinged theater under the fake stars.

The idea definitely didn't come from Roger Waters. In an effort to shatter the band's "space-rock" image, the lyricist had taken pains to write what he believed would be concrete, easily understood lyrics on common concerns. Audiences, of course, heard things differently.

To make matters worse, the album's celebrated quadrophonic mix was played in stereo over a lousy PA for the assembled record execs and critics. Of the Floyds, only Richard Wright bothered to show up.

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