Musical Epiphany
Of Styx, at least.
I've always been a big Styx fan. But tonight while screwing around on the guitar, I happened to figure out the song Renegade. Four chords, grimace musically. So I went on to a few other songs (Blue Collar Man, Heavy Metal Poisoning, Fooling yourself) in about 20 minutes.
I don't think my guitar playing has improved that much, so I can gues that most of the original riffs on this were written in about 30 minutes. Stick with either D, F, or A minor and you have at least those four figured out.