The café wall illusion is an optical illusion, first described
by Doctor Richard Gregory. He observed this curious effect in the tiles of the
wall of a café at the bottom of St Michael's Hill, Bristol.
This optical illusion makes the parallel straight horizontal lines appear to be bent.
To construct the illusion, alternating light and dark "bricks" are laid in staggered rows. It is essential for the illusion that each "brick" is surrounded by a layer of "mortar" (the grey in the image). This should ideally be of a colour in between the dark and light colour of the "bricks".