Great question — answering as the UphillTech team:
Use a paper clip - If the zipper is not stuck and the issue is just with a pull tab missing or not being functional. Insert it into the area where the pull tab attaches with the zipper slider and it will perform just like the pull tab. For zippers that need a lot of force, use proportionately stronger paper clips. I have used this successfully for opening suitcases that have missing pull tabs.
Full answer by planetregin: https://lifehacks.stackexchange.com/a/9344 (CC BY-SA 4.0).
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