Look at the parallelogram below. It is divided into two parallelograms of different sizes, each of which is crossed by a diagonal line painted dark blue.
Of those two diagonal lines, which is the longest? The one of the small parallelogram on the right or the one of the larger parallelogram on the left?
What you see, it is an optical illusion that was described by the German psychologist Friedrich Sander in 1926. The diagonal that crosses the larger parallelogram seems much longer than the diagonal that crosses the smaller parallelogram. However, both have exactly the same length.
The explanation of this effect is based on the fact that the diagonal lines give us a depth effect similar to the perspective, and therefore they are perceived of different lengths.