Look at these two lines. Doesn't one appear to be longer than the other? Is the one on the bottom actually longer than the one on the top or is it a trick of the eye? No, your eyes aren't deceiving you, they're different lengths.
Stare at this circle for a minute. Actually several minutes. Seeing anything? Perhaps several minutes more. Excuse me while I go to the bathroom. I'm back. Still staring? Be patient, this could take hours. Still not seeing anything? Perhaps something is wrong with you.
Look carefully at this square. Now close your eyes. Now open them again. Now close them again and then open and close them again. Look at the square again. Notice anything different? No? That's because a square is not an optical illusion, no matter how much you open and close your eyes.
Try this. Study these two arrows. Do you see a third arrow? That arrow is not really there.
Look at this perfectly straight line. Does it appear all wobbly? It isn't! That's just your eyes playing tricks on you.
Look at this semi-circle. Do you notice anything extraordinary about it? Is it even there at all? It's all about your perception of reality. When I see a tree, I ask myself "Is that tree really there, or has this whole world been created in my mind just for me?". I am constantly questioning. Maybe you should do a little more of that.