When my wife and I were dating in college, I would go hang out with her and her roommates a lot. One day I had a class that was cancelled (okay, I skipped). So I showed up at their apartment and noticed that one of my wife's roommates (we'll call her "Eva") had left her keys in the door. I figured it was an accident, so I decided to mess with her. Since they weren't expecting me to be there that early in the day, I grabbed her keys and moved her car around the corner out of view. Then I waited about 10 minutes for good measure and knocked on the door.
And this is where the beauty of a good prank is… in the details. It's too easy to say, "Hey, someone just drove off in your car." And it would be too obvious. You have to let the prank recipient come to their own conclusion. It's a lot like what they did in Inception. You have to make your subject believe the idea was their own.
So as soon as "Eva" answered the door, the following exchange took place:
Me: "Hey, did I just miss your brother?"
Eva (confused): "What do you mean?"
Me: "Lloyd. Is he borrowing your car again?" (It took me ten minutes to remember his name.)
Eva (looking at her empty parking spot): "No, he wasn't here. But where's my…."
That's when "Eva" looked around the parking lot and stared into space for a moment. Then she suddenly looked at the doorknob and clapped her hand over her mouth with a gasp. So I had to keep playing my part:
Me: "What? What's wrong?"
Eva: "I left my keys in the door!"
Me (holding back a grin and acting thoroughly confused): "For Lloyd?"
Eva: "No. Oh no! Oh no!"
Then she bolted into the apartment and told her roommates that someone stole her car. And since I hadn't let either of them in on the joke, they reacted perfectly and they went into a tizzy over what might have happened to her car. I let her freak out about it for a few minutes (while everyone kept saying "oh no" and kept looking up and down the street with both hands on their heads). And as soon as she picked up the phone to call the police, I handed her the keys.
She learned a valuable lesson that day (don't live with anyone who would date me). And while I lost a friend forever, it was totally worth it. You should have seen her face!