The theory that, no matter where you are, if you shout out "IT'S THE FINAL COUNTDOWN!" someone, somewhere will repond with "Duh duh duh duh...duh duh dut dut duh" I.e. The sound of the music which immediately follows the lyric.
John: Hey Mark, wanna try out the final countdown theory?
Mark: But we're in an old people's home!
John: So what, it'll still work!
Mark: Alright fine then, "IT'S THE FINAL COUNTDOWN!"
Voice in the distance: Duh duh duh duh...duh duh dut dut duh
John: HOLY SHIT OLD MAN, YOU ARE JUST THE COOLEST.