Subject: why does the sound made by a spoon 'clinking' a cup of coffee rise in pitch

I noticed, as a teenager, that the pitch created by a spoon 
contacting a coffee cup while making instant coffee rose as I stirred 
it. Figured that it probably had to do with heat from the boiling 
water I'd poured into the cup acting on either the cup or the 
spoon... or the level of the liquid rising in the cup as a result of 
the stirring; reducing the "free vibrating" surface area above the 
liquid level. Tried the same procedure w/out stirring, w/ a cold 
spoon tapping the outside of the cup, w/ hot water only (in case the 
density of the coffee might be a factor)... isolated it to the cup. 
Tried just pouring boiling water in a glass, tapping the outside w/ a 
cold spoon, similar effect. My best guess is that the ceramic glaze 
on the cup (being, for all intents and purposes, glass... I reckon) 
attempts to expand as it is heated by the boiling water, but is 
constrained by low elasticity? Resulting in increased internal 
tension? Raising the frequency of vibration-- much as tightening a 
guitar string does? My life took me in directions far from furthering 
this investigation, to a career in the Air Force, and now to England, 
so I have never gone any further in my quest to answer this nagging 
question. I have posed it to everyone I've met w/ a background in 
physics or frequency propogation, but have never had it 
satisfactorily answered. Stumbled across your site tonight, and 
thought I'd give it one last go. Here's hoping...

Re: why does the sound made by a spoon 'clinking' a cup of coffee rise in pitch