Students at USC's video game design school say jumping and flying are the fun experiences that keep us coming back to classic games. - 

It's just a week to Christmas, and odds are, someone on your list wants a video game.

Typically, they want the latest installment of a blockbuster franchise -- Halo 4, Just Dance 4, and on and on -- games whose sales equal, or even surpass, the box-office receipts of blockbuster films. This year you can add to that list Super Mario Brothers, which has sold over 40 million copies worldwide since its release back in 1985. And that's just the original version. If you add up all the iterations of the Mario franchise, it's more than double that.

In an attempt to understand why these games keep coming back, and why we keep buying them, we paid a visit to USC's School of Interactive Media Division to play some games and pick the brains of Sean Bouchard, Elizabeth Swensen, and Simon Wiscombe. Sean and Elizabeth are grads of the program and they've returned to create games for a living. Simon is a third-year grad student.

We put the question to them: how do you design a game that's "addictive"?

"Every game manipulates your behavior, that's the secret of game design," says Bouchard. "Movies express things through dialogue and images, we express things by behavior modification, really, by telling you what sorts of behaviors we will reward and what sorts of behaviors we'll punish."

Follow Kai Ryssdal at @kairyssdal