I washed a big picture window downstairs and found it wouldn't come clean, even after being washed from both sides. (I know that trick, thank you very much.) The dirt that's there looks like it's somehow on the inside, between the window panes where you can't get at it. I googled something like "dirt between windows" and found out that the seal on the window is probably broken. The gas that's put between the panes when they're installed has leaked out and air and dust are getting in there. It happens to old windows, they say.
Which brings us to our next question: how do you tell how old your windows are? In this case, when J was here, she very helpfully pointed out that windows often have birthdates on them in tiny print along an edge. Sure enough, this dirty window was born in 1980. Seals are supposed to last 15-25 years, so this is a window that has stretched itself to the very end. Or the almost end, because we probably won't replace it until next year sometime.
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