Finally it had arrived, his construction set for the square wooden frame. Its apparent simplicity made his heart pound faster. Four wooden pieces. All shaped exactly the same. What a lovely sight. He could hardly wait to see the resulting frame on his table. So many ways to use it. So many ways. Maybe he should have ordered more than one.
Putting the frame together was not as easy as he had expected. Joining two pieces by way of the double dovetail connection was just as impossible as separating them. He considered himself fortunate when he realized that he possesed an old puzzle magazine that revealed the secret behind the double dovetail connection.
Still, the manual that came with the wooden frame clearly showed that you needed all four pieces. Four? What had he done wrong? He went over it once more. No, there was nothing wrong with any of the connections. All angles were orthogonal. As they should be. How then had he managed to make a closed loop from only three pieces? He didn't know. He didn't even want to know! Maybe it was best to drink some coffee first. But isn't it always?
This image has been rendered using the POVRay raytracer. I created the original scene file more than ten years ago, but recently made a few improvements to it, and rendered it again at a much higher resolution.
Note: No post-processing has taken place on the image to make the illusion work; this is the 3D scene as it is rendered by the raytracer. "How can this be?" you may wonder. That's a good question indeed.