I love this illusion. Squares A and B are "clearly" of different shades of grey, eh? So why do when I connect them with a line with A's shade (or B's shade), a line which is in fact of one shade because I selected it that way (with Paintbrush), the lines lighten or darken depending on which square that it is in.

Of course you're brain is fooling you. Lesson? Take caution listening to your brain!