Drawing — I created these hand-drawn stereograms at age 14. They are not the greatest works of art, but hint at my future tendency to push boundaries.
Stare beyond the screen or cross your eyes until four images appear. Then allow the images to converge to a set of three, focusing on the center image, where the three-dimensional space will emerge. If the display on your screen is too large, most browsers will allow you to zoom out with ctrl-minus or option-command-minus. Return to normal with ctrl-plus etcetera. More info about stereoscopy via wikipedia.