Illusion: Lying down on a hard flat floor move arms up and down (as when doing a snow angel) while keeping them touching the floor. While repeating it a few times the floor begins to appear concave, with the deepest part where the arms as perpendicular to the rest of the body.
Questions: What is the origin of this illusion?
My guess: I think it might be related to the arms being harder to extend towards the back when they are perpendicular to the body than when they are at other angles.
Note: The name above is only the way I like to call it. I found this illusion when I was a kid, but I am sure others might have noticed it as well.