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Backtracking: An Interactive Tessellation Game
Click on the squares to move the tiles and create the correct pattern. A fun way to explore the concept of tessellations.
Wolfram Research
Wolfram Math World: Semiregular Tessellations
This one page Mathworld site has a number of examples of semiregular tessellations. A definition and a small amount of related information is given at the bottom of the page.