Eli5: Why does water that dries on glass leave “streaks” or grey markings on the glass?
Water is almost never pure. It always has a little salt and minerals dissolved in it. If you just cleaned using that water, it could also have bits of the mess dissolved in it.
Any time water dries, it’s going to leave this residue. Glass is special because it’s VERY smooth and reflects light like a mirror. Any time something gets stuck on a glass surface, it’s easy to see. It reflects totally differently than the glass, and pops up from the surface when nothing else does.
Because what we think of as water is not only water. It has minerals and other things in it that do not evaporate
The water has stuff in it. When it dries, the water turns into a gas and leaves whatever was in it behind.
It’s usually leftover minerals that were dissolved in the water, but they get left behind when the water evaporates.