42 is not the only answer!!!

'X’ is an optical illusion first published as a rhomboid in 1832 by Swiss crystallographer ‘Y’. ‘X’ is an ambiguous line drawing. The effect is interesting because each part of the picture is ambiguous by itself, yet the human visual system picks an interpretation of each part that makes the whole consistent. ‘X’ is sometimes used to test computer models of the human visual system to see whether they can arrive at consistent interpretations of the image the same way humans do. Humans do not usually see an inconsistent interpretation of ‘X’. ‘X’ whose edges cross in an inconsistent way is an example of an impossible object. ‘X’ is used in epistemology (the study of knowledge) and provides a counter-attack against naïve realism. Naïve realism (also known as direct or common-sense realism) states that the way we perceive the world is the way the world actually is. ‘X’ seems to disprove this claim because we see one or the other form of a cube, but really, there is no cube there at all: only a two-dimensional drawing of twelve lines. We see something which is not really there, thus (allegedly) disproving naïve realism.

'X'?

'X' is basically the cube that you draw on paper. It's called a Necker cube.