The photograph is of some visual illusion paving stones found in Bogot√°’s Zona T this morning. They give the impression of an uneven surface despite being completely flat.
I was in Bogotá to give a talk to the Asociación Colombiana de Psiquiatría Biológica who kindly invited me to their Christmas meeting.
Many thanks to them, and to the town planners of Bogot√°.
Mind Hacks 
I really like the visual effect of the paver blocks. Being a wood turner I enjoy finding these illusions in blocks of wood I turn.
One such block is at the URL above (click name). I’ll add, because this is my first post, Mind Hacks is a wonderful site and an every day read.