Like the original Kanizsa triangle introduced by Gaetano Kanizsa, an Italian psychologist. Here is another Kanizsa Girl. The basis of the Kanizsa illusion is that we perceive what is not there based on what is there. First we perceive a triangle on the left then a square on the right even though they don't exist.
Now many of us will look at the below image and perceive the woman that may have been in this bed based on what we see even though she isn't there. This was from the Imaginary Girl group on Flickr.