Freak out your coworkers or roommates by putting the cat in the microwave!
Not a real cat, of course, but a colour printout taped to the inside of the microwave door with a flashing light mimicking an electrical zap. Make sure to stay nearby with a camera on hand to capture the look on your victim's face, and to assure them there's no cat inside the microwave.
*It's never okay to put anything other than food in the microwave, so don't do this with a real cat (duh). Also, don't turn on the microwave with the flashlight inside.*
Step 1: Search and Print
Use your favourite Internet search and look for cats (or any other animal or thing you think would be fun in the microwave for the prank), then print them out in colour on your home printer.
Print them out as large as possible, most print options will have a "fit to page" button that you can select that will fill the image to the printer page.
Step 2: Tape to Inside of Microwave Door
Using a transparent tape applied to the back edges of the printout the paper is attached to the inside of the microwave door, making sure to tape the edges flat against the door to eliminate any gaps.
Step 3: Add Flashlight
Since the inside of most microwaves are white it acts as a nice reflector for light to bounce around in. Place a small flashlight inside the microwave chamber pointed towards the back, that way the light bounces back to the taped on image.
This small camping flashlight has a strobe feature, which adds to the effect.
Step 4: Close Door and Wait
Close the door of the microwave (do not turn on), and wait for someone to walk into the room. Since it's a silent prank it may take a little while to notice, but this just adds to the surprise.
Of course, a prank is only fun if the person is let in on the joke - so make sure you're close by to open the microwave and show them there's no cat inside.