Why when girls aren't thinking about it they open their mouth when putting on mascara?

Answer 1

If there is any real physical help in opening the mouth, it would be to pull the skin taught under the eye. With the skin pulled tight, she is less likely to accidentally get mascara on the skin, since that would make a mess and she would then have to wash it off and start over.

But one must wonder, is this small effect really enough to make any difference? Perhaps, perhaps not.

Answer 2

Some people would argue that the slack jaw is a completely mental thing, to help the woman concentrate on the application, much as someone might stick there tongue out of there mouth to concentrate more on a video game. It doesn't really help physically, but mentally it might actually make a difference if it helps them to concentrate.