One row of tiles seems to converge to the right. The row just above or below it seems to converge to the left. The effect seems strongest away from the center of vision.
When the graphic is shown in its pure form, you can sense the lines separating or constricting in your peripheral vision as you scan each row from the right to the left.
This building in the Melbourne docklands was designed to take advantage of the illusion. It takes force of thought to convince yourself that the floors are really level.
Here’s a technical explanation for the cause of the brain teaser, according to Michael Bach (link)
This illusion demonstrates the effect of some simple image processing occurring at the retina combined with some complex processing in the cortical cells of the striate cortex. The incoming image is first filtered by the centre-surround operator of the retina. The apparent tilt of the mortar lines is caused by orientation-sensitive simple cells in the striate cortex. The cells interact with one another to interpret the diagonal bands produced by the retina as a single continuous line, tilted in the direction of the diagonal bands.
Not sure I got that. But the café wall illusion should remind us as artists that we don’t see as a camera sees. Our brains actively construct reality, and that construction process is occasionally fallible.
Wikipedia on the café wall illusion: link.
Michael Bach’s explanation, with an animated figure.