It all began in the last quarter of 2012. Apple's (NASDAQ:AAPL) gross margin declined by a whopping 600 basis points year over year. The trigger? 80% of revenue during the quarter came from products that didn't exist 60 days earlier. This is the main reason, according to Tim Cook -- but there's more to the story.

In the video below, Fool contributor Daniel Sparks breaks down the backstory on Apple's margins, and he explains to Motley Fool analyst Rex Moore what investors can expect going forward.