Impossible things

The game featured on Tuesday - Echochrome - owes much to Oscar Reutersvard (Wikipedia entry) and M C Escher (Wikipedia entry) who were both fascinated by perspective and how 2D drawings of 3D space can lead to unexpected results.
 
Oscar Reutersvard produced so many drawings that he is credited with creating the category of imaginary objects called - Impossible Objects. There is a very large gallery of impossible objects at the 'Impossible World' site.