Slim, the custodian, thought his spring cleaning would be a breeze, but ghostly smudges kept appearing between the tiles of his shower wall. Was someone playing an April Fools' prank on him?

The Mystery: Slim had been scrubbing tiles all morning, but every time he stepped back to admire his work, he'd spot a smudge -- then another, and another -- where the white lines of grout intersected. Then he noticed something peculiar: if he stared at any one smudge, it would vanish, only to reappear when he looked elsewhere on the wall. See for yourself. Can you spot the tricky dots that are making Slim's job a chore?

What's Going On: It's not an April Fools' prank, but a very convincing optical illusion known as Hermann's grid. The tile pattern plays with our peripheral vision, causing us to see ghostly images where the white lines come together. However, when we stare directly at one of the dots, we use our central vision, which is not so easily fooled. For a more detailed explanation of how this illusion works, visit pbs.org/wnet/brain/illusions/color.html.

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