This three-dimensional recreation of one of artist M.C. Escher's best-known drawings is best viewed full-screen so you can clearly see the water flow. (Click the little symbol at the lower right of the video.)
Notice anything odd? (If not, you might want to switch to coffee.)
There's a separate explanatory video that points out the clues to what's really going on. No computer-generated imagery is involved. If you were there in person you'd see the exact same thing, provided your eye was in the position of the camera lens. It's all a matter of practical special effects (hint: there are actually two streams of water) and optical illusions, and it works only from this one viewing angle. Very cleverly done.
The late American magician Jerry Andrus created many similar illusions. You can view a nice collection of clips about him and his creations from broadcast television in the early 1980s.