system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some elderly people move their mouth a lot, as if they're chewing on something?
gpt
The chewing motion is found almost exclusively in people who have lost teeth. On rare occasions, certains tranquilisers or antidepressants  may cause a side effect called tardive dyskinesia, an inability to control what are ordinarily voluntary movements. These movements are as likely to involve the nose as the mouth or jaws, though.

In most cases, the chewing motion is a neuromuscular response to the lack of teeth: an attempt by the oral cavity to achieve some from equilibrium. In particular, sufferers can't position their upper and lower jaws properly. With a full set of ivories, the teeth act as a stop to keep the jaws in place.