Which square is darker, A or B?
The answer is that both squares are the exact same shade of grey. The connecting line in the image bellow allows your brain (for the most part) to see that they are indeed the same. This illusion was first published by Edward H. Adelson, Professor of Vision Science at MIT in 1995. Here you can find more information including an explanation of the effect.
Here is an amazing real world version on the illusion.