This Instructable will show you how to make a simple optical illusion. This illusion is called the Poggendorff Illusion and has been around for a while.
So what's happening in the image below? What is that red line doing there?
Go to the next step to see what's going on inside the black rectangle.
Step 1: Inside the box
Here you can see that the blue line actually leads into the red line. Sure, the color contributes to the illusion, but it can be confusing even if the lines are all black.