I'm not quite sure how this works, but I love it. It's Andrew Lipson's version of MC Escher's Ascending and Descending (that's the one where they are going around in a square but upwards all the time) but done with Lego!

Presumably it has to be photographed from exactly the right angle.

Ah. It's all explained here.

I rather think you may have seen this before and that I'm the last one to notice. Oh well.