Algodoo

Look at the shapes on the cover. Do you think the red line is longer, the blue line is longer, or are they the same length?
















I guess many people would answer 'the red line is longer,' but actually, the two lines are indeed the same length. If you don't believe it, you can delete the two gray diagonal lines and take another look.

The human eye is very precise and quick at interpreting 2D shapes as 3D objects, so it's easy for us to see these two gray lines as a road stretching off into the distance, and then, following the principle that things appear smaller when they're farther away, incorrectly conclude that the red line is longer.