Of course, you'd only use this knowledge for good...like when you're smart enough to remember how to open a door with a credit card, but not smart enough to have remembered your keys in the first place. And be warned, it takes alot of work, and will ruin your card, so use an old gift card or store rewards program, which can always be replaced.
1. Slip the card into the vertical space between the door as deeply as you can, as perpendicularly to the door face as you can.
2. Then, "tilt the card so the side that's closest to you is almost or is touching the doorknob. Push it until you feel it slide in more."
3. Lastly, bend the card the opposite way, forcing the the lock to go into the door, and quickly open.
Read all the details from WikiHow. Via.