In this image, the green and the blue, in their respective spirals,
are the same color. Exactly. It doesn't look that way to me,
but it can be demonstrated in a couple of different ways. See:
This math puzzle, obtained from
http://www.briandela.com/files/picture.gif (no longer there),
is very interesting. Note that this works even if you print this,
cut out the pieces, and do it on your desk. Or even if you make
your own pieces with the appropriate dimensions.