why can't women put on their mascara with their mouth closed?

I am one woman who puts her mascara on without opening my mouth. It is a matter of using different facial muscles. I always thought it was strange that women opened their mouth to put on mascara. I also do not curl my eyelashes. I think that many women open their mouths because that is they way they learned to do it watching moms or older sisters doin it. Good question.

As a young woman I just do it naturally. I've tried putting it on with my mouth closed but then I get the giggles. I don't know why lol, I just do. I gather there are some things that we, as people do that are unexplainable, even to ourselves. Just let nature be.