These three pink boxes sit on a blue
and yellow checkerboard pattern.
If we add shadows underneath the boxes,
they seem to float.
If we move the position of the shadows,
the boxes seem to float even higher.
That's why you have to beware of the shadows - they can be deceptive.
I found the original shadow illusion on Brad Honeycutt's blog on optical illusions, called, oddly enough, An Optical Illusion. I modified it a bit to show the boxes without shadows.
Floating Boxes Optical Illusion>>