If you find the screws on your sunglasses always go loose then once you have tightened them up, apply a tiny dot of clear nail polish over the screw, let dry and walla!
...and, if it's too late and you've already lost the screw, a piece of wire from bread ties, etc. works very well in it's place. Make sure you strip the paper or plastic from the very thin wire inside, put the wire through the holes where the screw was, and tighten it as much as you can. Once you break off the excess, you can't even see it. I did this for my daughter's glasses, and you can't tell it's there, it's so fine, and it works so much better than the screw that wouldn't stay in (too bad I didn't get the polish tip earlier!).
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