How many prongs does the fork have?
Concentrate carefully and take a look at this image. How many prongs does the fork have?
If you just take a quick look, this looks like a normal fork. But this fork can’t exist in the real world.
That becomes clear when you cover first the left, and then the right half of the fork. When the left side is covered, you can see a rectangle with two prongs. When you cover the right, you can see three round prongs.
Oscar Reutersvärd, a graphic artist from Sweden, and Maurits Escher, a Dutch artist, have designed many more mathematically impossible images and structures like this.