Subject: Why is that when you add water to a glass and tap it, the pitch decreases?

Originally I thought that when you tap the glass the air inside would 
vibrate and resonate at its natural frequency (like it does why you blow 
across the top of the bottle.)  In which case, I would expect there to be a 
higher pitch when there is less air.  But the opposite occurs.  I thought 
maybe it was that its the water and not the air that resonantes when you 
tap it (or maybe just that the water is vibrating, and not resonating, at 
its natural frequency).  But then I realized that there's an even higher 
sound when you tap an empty bottle (or at least an equally high sound as 
when you add a little water).  I read explanations that say that the glass 
is vibrating and when you add water, you're increasing the mass of the 
system and a more stable object vibrates slower when its struck.  But I 
always learned to consider natural frequency as a function of what 
wavelength is fitting in it.  And I don't know how to connect those two 
ideas.  Thanks.

Re: Why is that when you add water to a glass and tap it, the pitch decreases?