Our brain can easily understand 3D images but have some trouble perceiving images in 2D. As a result, our brains try to use the same rules that apply for 3D images as for 2D images. In the first picture, though the second line looks smaller than the first however, that is not true. The depth perception rule explains that the retina is able to pick up the image and is not fooled but, when the message is sent to the brain, the brain processes it as looking shorter because of the way the arrows face is congruent with 3D objects we see daily. For example, when we look at a building, we see that the outside corners of the building as close to our position and we see that the inside corners of buildings as meaning that it is far from us. To test this theory they studied africans who live in circular huts and found that they were not fooled by this illusion like we are.