My rear brakes squeal like crazy, but not when they are wet! I am a new cyclist in Bonn, Germany. Someone gave me a free bike that is very nice, but the rear brakes squeal like crazy, and the weird thing is that when it rains and they are wet, they don't squeal at all, which is the opposite of what I have read, since often people's brakes only squeal when they are wet. What could this mean? Any ideas of how to solve this problem, other than pouring water on my brakes on purpose? Lol! The brakes are new and they work fine, just to give you more information. Also adding that they are rim brakes.

I know a lot of people don't care about squealing brakes, but I mean we're talking, if rode my bike past 11 pm I could wake up entire apartment buildings! And I seem to be the only person in Bonn with this problem, so I'm not going to lie, it's embarrassing. I definitely get looks too! I could just get a new bike but I guess I'm wondering if there is possibly a simple solution. I already asked a bike repair shop and they didn't think anything could be done, but that was also before I had this piece of information that they don't squeal when wet.

They are rim brakes.
You probably need to toe in your brake pads. That is, adjust them so that the end of the pad closer to the front of the bike touches the rim slightly before the back end of the pad.  There are plenty of videos on YouTube explaining how to do this, and it's not at all difficult. The quick summary is that you loosen the brake block's retaining bolt slightly, then apply the brakes with a piece of cardboard between the back of the brake block and the wheel. Then tighten the retaining bolt back up again, and repeat on the other side.

The squeal is caused by the reverse set-up, where the back end of the pad contacts first.

To see an example of what's going on, hold your hand flat, a few centimetres above a slightly rough surface such as a wooden table, and tilt your hand downwards so that your fingertips are pressing quite hard against the surface. If you move your hand away from yourself (so your fingertips are at the front), you'll feel a vibration as your fingertips alternately grip the surface and release from it; if you move your hand in the opposite direction, you'll feel it slide much more smoothly.  What's happening is that, as you move your hand away, your fingertips are being pressed harder into the surface, which increases the friction force until it gets too much and they slip and jump. When you move your hand towards yourself, the movement doesn't cause your fingertips to be pressed harder against the surface, so you don't get this alternate slipping and jamming.

It's this repeated jamming against the rim that causes the squeal.  Why are they quiet in the rain? Because the wet rims have much less friction, so the forces don't build up nearly as much.