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When using a new disposable mask(as is common nowadays due to the COVID-19 situation), are you allowed to pinch the wire to make it fit your nose? It seems to me that it should be considered מכה הפטיש( finishing  a device or vessel) and therefore forbidden. Did anyone hear any halachic opinion on this?
 

That's a great question!  Since it's formed again each time you put it on and you press it flat again while taking it off it's probably allowed but maybe we should stick to masks that either don't have the metal or was shaped in advance.

When I was using them, they retainer the shape after the first use.

PS Initially I didn't pinch them at all; however, someone showed me that this was the reason my glasses were getting fogged up. Once Iearned how to make right kind of pinch, the problem went away.