Walls do more than just hold up roofs. They act as a complement to the furniture within a room. You can have painted walls, plain brick walls, or wallpapered walls. But no matter how far wall technology develops, it is still impossible to hang a picture up straight first time. Every time I try to put a picture up, the whole wall turns itself on an angle temporarily, only to make it obvious when I back away that a disorientated blind polar bear hopping on one leg down a cliff-face could have done better at hanging it straight.
Another pitfall is the dreaded TPAGF, the Temporary Picture Anti-Gravity Field, which will cause one side of the picture to fall two inches the moment you let go of it. Scientists believe this is related to the strange behaviour of the hammer being attracted to your finger when you try to put a nail in. And to the inability of anyone to steer a shopping trolley.