Why does your lip quiver involuntarily when you begin to cry?

Question

Also, why only the lip in particular?

In:

Answer 1

It’s part of the involuntary ‘body language’ that lets other people know how you’re feeling.

As social animals, humans have evolved many traits like this.

The ‘why’ of it is just simply the way it is. There’s no underlying logic to only the lip in particular, just that it is a facial feature with a large variety of movements.