© 2007 Kingdom, Yoonessi & Gheorghiu

Here is two pictures of the Leaning Tower of Pisa,
but the two pictures are exactly the same, ie, it is one and the same picture.
However, it seems that the right tower is inclined more than the left as photographed with another point.
The illusion consists in that our brain perceives the two images as if they were part of the whole.

Typically, two parallel columns, standing nearby, we perceive converging to a single point on the laws of perspective, as in the case of the columns of the temple of Artemis in Jordan, that you see in the picture on the right.

Temple of Artemis in Jordan

The illusion works well also with other scenes, such as roads or railway tracks, stretching to the horizon. Examples can be found HERE.
Try to experiment themselves.