Subject: Why can you see your fingerprints on the side of a glass

If a glass of water is peered into from above all that can be seen is the 
inside of the glass. The sides of the glass reflect what's inside. If you touch 
the outside of the glass, suddenly you can see through, except in the valleys 
of the finger prints which still display a reflection. This only works if the 
outside of the glass is wet. What physics are involved? Why doesn't the side of 
the glass just continue to reflect the inside of the glass?

Re: Why can you see your fingerprints on the side of a glass