In his later years, my grandfather took to intricate wood carving and burning, and he used one of these magnifying headband visor to see all the details. The siblings and cousins and I would sneak into the workspace and put it on, and then look down at the floor with our skewed depth perception through and below the lenses, making a seemingly ten-foot vertical drop off, into which we would fearfully step and then giggle when we’d realize we hadn’t fallen.
These fantastic faux-floor illusions highlighted at BoingBoing create a similar but much more terrifying effect…and are way more awesome. I wish my grandpa had taken to this as a hobby.