[url=http://gizmodo.com/holy-crap-these-four-images-are-actually-just-one-singl-525258272?utm_campaign=socialflow_gizmodo_facebook&utm_source=gizmodo_facebook&utm_medium=socialflow]Source (Gizmodo)[/url]
If you stare at it long enough and really concentrate on connecting everything together, you can tell it's a single image. Very tough though.
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I'm calling bullshit on anyone who says it's obvious. Hard boundaries, like the ones seen in this picture, rarely exist and never divide a region evenly.