Fourth graders are using this website to view optical illusions. This site will tickle your brain, too:
This student is demonstrating the "floating finger" illusion from the book Amazing Optical Illusions.
You can find optical illusions in the Rose Park Library in the nonfiction section at 152.14.
Use the Order in the Library website to practice putting books in Dewey Decimal order:
When you are at the Order in the Library site, choose the Shelving game. Then choose Dewey Decimal.
Start with Expert and see if you can work your way up to Super Genius.