This prank is genius. Thanks to the wonders of optics and a simple Photoshop technique you can easily make a jar with a pickled human head inside. This will make anyone scream in the office or at home as soon as they open the fridge—such a great effect.
You only need to make a continuous graphic by taking two photos of your face from the front and the side, and then merge them in Photoshop like this:
Then print it out, fill the jar with tinted water (make that tint acid-free to maintain the print in a good enough state for a long time), and put the print inside.
Andrew Liszewski's tip for extra realism: take the front photo with your face against a glass, so you get some deformation on the nose!