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18.8 Riddles & Brainteasers
Thinks.com. The main page also has crosswords, word searches, and other fun things.
this is probably the best one. It's from Ritsumei.
There are a lot of optical illusion sites on the web. There is lots of overlap between them. Here are just a few of the better ones:
Many more are easily found using a search engine.
Some artists have used optical illusions in their work. M.C. Escher is the most well-known.