Every neighbourhood has a few: windows held open with a stick, a hardcover book, or a strategically placed spray bottle. It works, which is why the stick stays there for years. But the window is not broken in any deep sense, and the actual fix is smaller than most people assume.
Meet the balance
A double hung sash is heavier than it feels, and the reason it feels light is a pair of hidden mechanisms called balances, one on each side, tucked into the frame. Each balance carries its share of the sash weight so the window floats: easy to lift, and staying wherever you leave it. Older windows do the same job with cords, pulleys, and cast iron weights inside the frame; newer ones use coiled springs or block-and-tackle assemblies.
When a balance stretches, snaps, or loses tension, the sash has nothing holding it up. Depending on how it failed, the window slams shut the moment you let go, drifts down over a few minutes, or lifts crooked because one side still has support and the other does not. Sometimes the failure announces itself with a bang or a twang from inside the frame.
Why it is worth fixing promptly
Beyond the daily annoyance, an unsupported sash is a falling object with fingers underneath it, which matters in a house with kids or pets. A sash riding on one working balance also travels crooked, which wears the tilt-latches and can rack the sash itself, slowly turning a cheap repair into a bigger one. And a window that cannot be trusted to stay open tends to just never get opened, which is a poor deal for a working window.
The repair
A balance replacement is one of the quickest jobs in window repair. The balance type and size are matched to the sash weight, both sides are replaced as a pair so the window travels evenly, and the sash goes back in square. With common sizes in stock, the whole repair typically runs under an hour per window. Older cord-and-weight windows get the equivalent treatment: new cords, serviced pulleys, and the original weights back to work.
While the jamb is open it is worth a quick check of the tilt-latches and the general condition of the window, since a sash that has been forced up and down on a dead balance sometimes carries a little collateral wear.
So retire the stick. The estimate is free, the part is small, and the window will hold itself up the way it was built to.