Remember how the bad guys in Scooby Doo would always use cut-out portraits to spy on people through walls? Well, unless your landlord is super cool, you probably shouldn't start cutting holes in your wall just yet, but you can make a higher tech version, thanks to NASA employee Mark Rober.
Mark used a cheap picture frame and a portrait with the eyes cut out to make the "Scoob Cam," which also doubles as a surveillance device. He used an iPhone and an iPad to start a FaceTime chat, then taped the iPhone to the back of the picture frame (where the eyes in the portrait are).
The iPad is used as the surveillance aspect, so when you watch the feed, it records your eyes and displays them on the iPhone in the portrait, so wherever you look is where the portrait seems to be looking.
The obvious downside to this trick is that it requires two mobile devices, but if you only have your smartphone, Mark explains a simpler method (without the surveillance) that uses his Digital Dudz mobile app to just give the appearance of moving eyes. Not as useful, but still pretty funny, and a good way to mess with your friends.
Check out the video for details on the setup.
For a similar, but cheaper prank, try this creepy motion-sensing portrait with glowing red eyes that light up when someone gets close. Or, if you do have two Apple devices, maybe you'll be interested in Mark's double-iPad beating heart costume.