system
Given the following post, respond as if you're commenting on Reddit:
human
What causes that squeaky noise when you rub a wet finger over a mirror or other smooth surface?

why doesn't it work with a dry finger?
gpt
There's two scenarios at play here: one where your finger is touching the surface, and the other where it's touching water. The surface has a higher coefficient of static friction, meaning it will do a decent job of keeping the finger in place despite other forces (it's sticky). The water, on the other hand, has a quite low coefficient of kinetic friction, meaning it's not good at slowing down objects that it's contacting (slippery). 

Let's look at a finger moving along the mirror at a constant-ish speed. Your skin is kind of springy, it stays in a certain place and the further you pull it away the more it wants to go back. To start, the skin is in contact with the mirror (pushing aside the water). When the finger moves far enough to the side, there's enough force to pull the skin off the mirror. The skin then glides over the water until it's past where it would be if it were sitting still. As it gets to the point where it's not moving past the mirror, it sinks into the water and grabs the mirror again. Then the process repeats.

This repeating process of sticking and slipping always takes about the same amount of time to repeat (with the same mirror, water thickness, finger force) and it happens pretty fast. Anything that repeats like this is going to vibrate the air around it and cause a squeaking noise. It's really the same thing as a violin, but instead of vibrating a string with a bow you're vibrating your finger with the wet mirror.

The pitch of the squeak is how many times your skin slips back and forth per second. It doesn't work with as well with dry surfaces because there isn't as much difference between static and kinetic friction, so your finger is dragged along constantly instead of skipping.