In the movie, whenever DiCaprio and his gang are about to wake up, they hear the song Non, je ne regrette nien by Edith Piaf. But you knew that part — they never hide the fact that they’re using that particular song as part of the plot. What you might not have realized is that you’re hearing it even when you think you aren’t.

Remember that ultra-dramatic instrumental theme you hear over and over for the last 45 minutes of the movie? It sounds like a typical summer movie soundtrack meant to let you know that shit’s getting real … but it’s actually that same Edith Piaf song, slowed down almost beyond recognition.

The really cool part is that it makes perfect sense. The Edith Piaf song is a way for the characters to know they’re about to wake up — but since time passes more slowly inside dreams, what seems like two minutes and 23 seconds in reality can last a lot longer for them. While the song is playing at normal speed in the waking world, the characters should hear it all slowed down.

Hans Zimmer, the film’s composer, said that in order to achieve this, they actually went to France and extracted two notes from the original master of the song. Apparently those two notes went a long way, because he also said that “all the music in the score is subdivisions and multiplications of the tempo of the Edith Piaf track.” So basically, old song + math = Oscar nomination.