There’s a sort of a reverse Thatcher effect also: have your friend lie down, with the lower part of their face (everything below the eyes) obscured with a piece of paper or something. You then make eye contact with them from above their head so that their face is upside down in your field of view. After a while, your brains starts to think you are face to face with someone without a nose or a mouth, but with a bushy beard (assuming they have hair).

(Discovered this with friends back when we were kids, don’t know if has a designated name.)