अजी ठहरो , जरा देखो: Why do girls cover their mouth with their hand while laughing and crying?

Maybe they have braces or got into the habit when they did and haven’t realized it’s no longer necessary.
Or, they need braces and their parents can’t afford them.
I guess it’s possible their oral cavity is unnaturally juicy so laughing risks spitting all over people or coughing up phlegm which if it makes it to their lip would be drooling.
Similarly, a hearty laugh during allergy season could inadvertently result in an unattractive nasal discharge, which is why the edge of their index finger of the covering hand always lightly blocks both nostrils.
It may just be to stifle an excessive laugh which can lead to various forms of leakage.
Maybe they have bad breath; or, smell like weed and don’t want to share.
They may laugh like a donkey’s bray and cover their mouth to stifle or at least muffle it.
It could also be they don’t find you funny but are faking a laugh to avoid hurting your feelings.

1 trying to act cute
2 trying to hide teeth
3 to show a control on laughing thinking laughing unappropriated for girls.
4bad breath
5 habit of shyness.

I don't know the particular reason behind. But as far as India concerned, I can tell you. When I was a small kid, everytime I used to laugh out loud , My grandmother used to tell me ' laugh but in lower tone covering your mouth with hand or some handkerchief '. She used tell me that 'girls should not laugh out loud like a man as it is considered to be mannerless. ' Hence I think most of the women in india cover their mouth while laughing. Some of them might be doing it just because others does it. I have seen men doing it too.