"Paranoid" (Black Sabbath)

I'm not even going to pretend that I'm a metal fan or know all that much about the band Black Sabbath.  As a music fan, I've heard the names Geezer Butler and Tony Iommi mentioned in passing for years, but I don't know enough to talk about their musical roots or playing styles, other than to use the word "heavy."  And the little I know about frontman Ozzy Osbourne (sadly) comes from The Osbournes 2000s T.V. show and the infamous, dubious lore about his stage antics.  It's still hard for me to imagine the shellshocked, soft-hearted patriarch of that reality show biting the head off any kind of bat/dove/porpoise/whatever it was he purportedly gnawed on stage.
No, my passing knowledge of the band comes from hearing their sledgehammer riffs flying at me through stadium speakers.  (Because nothing says "hit that home run" like the bludgeoning riff from "Iron Man" or "War Pigs.")
Same goes for the song at hand, "Paranoid," which comes from the 1970 album of the same name.  That riff is so chiseled in stone and full of adrenaline that it's undeniable.
Seriously, if you're not motivated to kick ass by the time Osbourne's voice erupts from the track, then you might want to go back and warm that bench.  Just saying.
I came across a 2010 interview with Butler via the Gibson guitars website (which is kind of interesting, considering Butler plays bass, but whatever), where he talked about how the track came about.  
In short, the song was an afterthought.  The album was essentially done, when their producer, Rodger Bain, told them they needed a short filler track to round things out.  Not having anything else in the bag, they came up with "Paranoid" in the studio, with Butler quickly writing some words for Osbourne to sing.  (The famously staccato manner in which Ozzy delivers the lyrics on the track is largely because he was still learning them.) 
Said Butler of the band's arguably most famous song: "Tony [Iommi] just played this riff, and we all went along with it.  We didn’t think anything of it."