What you Need
A good quality crystal wineglass that rings for several seconds after you ping it.
A little water
What to do
Fill the glass with about 1-2cm of water
Make sure that your finger isn't greasy,
Wet your finger and gently slide it around the glass. It should start to ring nicely.
Get it ringing really well and then stop. The ringing should die away over a few seconds.
Now do the same thing but while the glass is still ringing pour the water out.
What may happen
You should find that the pitch of the ringing drops as you pour the water out, but if you let the water run back the pitch will go back up.
You can try it as many times as you like, try varing the amount of water and the speed at which you pour it out. You could even try tilting the glass, then getting it ringing and straightening it up - you will find the pitch goes up.
Why does it happen?
When you move your finger around the glass it tends to stick, then slip then stick again which tries to vibrate the glass. The glass vibrates very well at a few frequencies in a similar way to a swing (if you push the swing at the right rate the swing will build up, at the wrong rate and you generally get hit in the face...). So if your finger is vibrating the glass at the right speed the glass' vibration will build up to the point where it vibrates the air, creating the sound that you can hear. This effect is known as resonance and the frequency it happens at is known as the resonant frequency.
The glass actually vibrates by deforming the rim first into an oval in one direction and then into one in the other.
Different glasses will vibrate well at different frequencies, in much the same way as different guitar strings will produce different notes. A glass with a large rim will vibrate at a lower frequency and therefore pitch than a smaller glass.
When you start to pour the liquid out of the glass the water moves over the part of the rim that is vibrating the most. This makes the rim heavier, and the added weight is more difficult to accelerate in and out, this means the vibration happens more slowly and so the pitch goes down. If you remove the water from the rim, either by pouring it out or straightening up the glass, the pitch will go back up again.
For the same reason strings on a bass guitar are made much heavier than on a normal guitar, as heavier strings vibrate more slowly allowing it to play much lower notes.