"Horse Illusion - 3D Stereogram" -- I absolutely HATE these things where you're supposed to stare intently until you see the hidden image. I can never, ever see them. Every time I come close, my brain snaps my eyes back into focus.
i am really good at spotting these 3D hidden images and find it easy, but i know some people just can't do it. i stare at it and relax my eyes like i'm nodding off and when it's real fuzzy, i cross them a bit. slowly.