A simple act of laying down is possible due to gravity. The gravity shapes our environment by pulling down all the existing living and artificial matter to the surface of the earth. We have evolved adapting to the gravity and shaping our environment according to it.
As a child, I used to play pretending I was walking on ceilings. Holding a mirror reflecting the surface above me, I walked from room to room, stepping over doorways. I was pulled down to the ceiling by mirrored gravity. If you think about it, reflections in mirrors are the exact opposite of reality.