The diagram illustrated shows an optical illusion using false perspective. The objects do not appear to be of different sizes depending on their distance from the viewer, as would be the case with true perspective.

The blue sphere is two levels higher than the red one, but this is not obvious. For the observer, both spheres seem to be on the same level. Accordingly, exactly one field is missing between the red and the blue sphere to connect them, which does not correspond with the opposite side and thus makes the figure impossible in perspective.

The same principle is used in M.C. Escher's artworks to create optical illusions and impossibilities.