Today, a single image that may leave you scratching your head for a few seconds as you try to figure out what's illusory about it.
(© 2005 Dejan Todorović)
If you can't get it, explanation is after the break.
If you look to the left of the arch, there appear to be three semicircular tubes right next to each other. On the right side, it appears to be a series of sinusoidal waves, or ripples. The different cutouts at the bottom help to cue us to see them that way. Without those cues, it would be ambiguous as to which one the shading is representing.
You may now go back to believing that the world is as you percieve it if you wish. Just know that you're lying to yourself.