I can't stop fooling around with our irrefutable certainties. It is, for example, a pleasure knowingly to mix up two- and three- dimensionalities . . . to make fun of gravity . . . . Are you really sure that a floor can't also be a ceiling? Are you definitely convinced that you will be on a higher plane when you walk up a staircase? Is it a fact as far as you are concerned that half an egg isn't also half an empty shell?