What is this 'Friends' background actor chewing on?

Background actors: Experts at mouthing fake conversation, laughing silently and avoiding eye contact with the camera. 

But all that faking gets repetitive and sometimes extras mess up. 

Take, for example, this Friends extra who appears to be chewing her coffee. Comedian Nick Turner noticed the background actor and posted a close-up.

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Naturally there are some solid theories as to what's going on here:

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A possible explanation, though there doesn't appear to be food on the table behind Ross. Of course, there could be something chewy in the cup itself:

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Or, you know, the camera caught this extra at a weird moment. It wouldn't be the first time Friends -- or any show really -- had some questionable background acting. Remember the one with the case of missing Jennifer Aniston?

[H/T: Storyful]

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