To best play this trick, you need to be wearing shorts and have access to a friend's camera when they're not around. Shorts weather seems behind us now, but spring is never too far away. Plus, this could be a good trick to play on a sibling, girlfriend, husband or whomever when they get up to go to the bathroom and leave their camera phone behind.
Step 1: Grab your friend's camera and bend your leg in front of you thusly:
Step 2: Zoom in on the crook of your leg so it fills up the view on the camera (in the photo below, the shaded parts of the photo would be outside the view of the camera):
Step 3: Take the picture and voila! Your friend will have a picture of what appears to be a random ass on their camera. Just let them try to explain that, especially when they email their photos to Wal-Mart to be developed!