Yesterday I went to drink out of a clear plastic pitcher at
the kitchen window.  I saw what looked like an oil slick on
the water.  Then I noticed that the colors weren't at the
surface, like they would be if it were oily.  They were on
the reflection of the pitcher--but there were no colors on
the pitcher itself.  Finally, I realized I was seeing the
thickness of the plastic pitcher.  How?

Light that is reflected from a surface is polarized parallel
to the surface.  So glare off a lake surface is horizontally
polarized.  Scattered light from the atmosphere is also
polarized; when the sun is on the western horizon, the light
at the zenith is N-S polarized.

This lets you make a natural polarimeter.  Put some
optically active stuff (cellophane works nicely) between the
blue sky and the water; then look at the reflection of the
cellophane in the water.  You'll see colors. 

--
Mike Stay
myturl.com/001p9