Here's a little do-it-yourself physics experiment for anyone who cares: Ask your parents for a crystal wine glass. Be careful not to break it. Set it on a table and hold it firmly by its base. Now, get your finger wet by dipping it in water and slowly drag it across the top of the wine glass in a circular motion. If you have found the right frequency, you should hear a decently loud ringing sound.
Why does this happen? Thanks to the delightful physics of resonance, we can understand this experiment. But first, here's a video of me performing the experiment.
(I'm not sure if this works or not but I took a video on my phone and emailed it to myself to attach as a file so I don't know if it shows or not)
When you rub your moistened finger along the edge of the glass, your finger sticks as it encounters friction. The water acts as a cushion to reduce the friction. When the friction is ideal so that your finger slips around the glass, vibrations occur on the sides of the glass. That vibration is then transmitted to the surrounding air creating a sound wave with a certain frequency, measured in per seconds (Hz). This is called a resonant frequency. A wine glass's resonant frequency is typically in the range of human hearing (20-20,000 Hz), allowing for us to hear the ringing.
Thanks for tuning in folks!