When you sit in the train and gaze at the distant horizon through all the trees that are flashing by, it is not perfectly clear whether or not those birds over there are actually floating on the water or hovering just above the surface. You see, the movement of the train combined with the perfect stillness of the horizontal line produces an illusion of depth. It is perfectly clear that the distant flatness is there and that the ground next to the window is there as well, however anything inbetween conveys less of such a distinction between this and that. The reflection of the blue sky on the water and the apparent shadows of the birds together make an unusual reorientation of things not that unusual at all. That is why it took my eyes a second or two longer to refocus and try to make sense of this interesting and misleading natural phenomenon.