The Exploratorium's 40th anniversary was last weekend and during that time on the Exploratorium website there was an online activity called "Surprise."
The activity was that you need to print out a paper and there is a little sign that has 40th in it. Next you need to go on the Exploratorium's website and click on the word surprise. Then put the paper in front of the computer and the 40th sign should be facing the camera and you will see a surprise.
Here is a video of how it's made. You may also want to watch Julian F.'s photo essay about the Exploratorium.