The Binding of Isaac has turned 2 years old, but it's gamers that get the birthday presents. For one day, The Binding of Isaac is $1 on Steam, an 80 percent cut from its usual $5 price tag. In addition, the creators of Indie Game: The Movie have released on YouTube a 30-minute excerpt from their documentary in which Isaac designer Edmund McMillen details the origins of his game.

McMillen explained Isaac was originally conceived as a game to be made in a week, while his friend and co-designer of Super Meat Boy, Tommy Refenes, went on vacation. "I didn't really have any idea of what I wanted to do, but I definitely wanted to do something risky," McMillen said. "I wanted to do something exciting, because I wanted something to replace that danger of running out of money, and the danger was, maybe I was going to do something that could hurt my career in some way."

Two years and 2 million sales later, it seems the game-in-a-week experiment worked.

This article was originally published on Joystiq.