| The missing square puzzle is an optical illusion used in mathematics classes to help students reason about geometrical figures. It depicts two arrangements made of similar shapes in slightly different configurations. Each apparently forms a 13×5 right-angled triangle, but one has a 1×1 hole in it. |

- Wikipedia at http://en.wikipedia.org/wiki/Missing_square_puzzle
- Michael's Blog at http://stickinsect.wordpress.com/2011/01/18/the-missing-man-and-the-missing-square-the-answer/
- Mark Wieczorek's blog at http://www.marktaw.com/blog/TheTriangleProblem.html

*(Note: None of these links are affiliated with Calculus Humor)*