So As Not to Be Surprised

Take a carnival mask, turn it with the inside towards you and have a proper look at that concave plastic shell in which the nose goes inwards, the cheeks sink in and the face is turned inside out like a glove pulled off a hand.

Now step back a couple of metres.

The mask is convex. An ordinary face, nose outwards, and on top of that it slowly turns to follow you as you walk sideways, although half a minute ago you were holding this shell in your hands, know absolutely everything about it and can say aloud that it is concave with the same firmness with which you say your own name.