Trompe-l'œil

Some optical illusions happen because we develop shortcuts in our visual reasoning, so that we can decide as fast as possible what we have probably seen, rather than taking a little longer to be absolutely certain. The evolutionary benefit of our brains working this way are obvious: you are more likely to live to raise children if you run for cover the instant you see a woolly mammoth, even if you occasionally waste energy and look like a fool by running from the shadow of a mouse

Observing babies (have I mentioned that we have grandsons?), you can almost see this development in action. The eyes becoming mature and the brain learning, oh so fast, to turn all those incoming light waves into meaningful information, then learning to store it and retrieve it and apply that knowledge. First you recognise, and remember, your mother's face, then you understand that similar-but-different arrangements of shapes - eyes, nose, mouth, hair - are also a face, that you can interact with in the same way. Some faces are so different that it takes a while to understand that it's not a woolly mammoth ("This is grandpa"), but when you overcome the illusion, the joy is transparent

And when they understand that they are looking at a face, then they look deeply and openly, seeing in you things that you did not know were there, and believing in you more than you believe in yourself, delighted by what they have found; an irresistible power

Make of this what you will