Girls, Why do girls giggle?

I was out shopping when I ran into a group of 4 girls around my age. I shared a glance with a few of them, but as I was walking out of the store they followed behind me, so I held the door open for them. They all said thank you and made eye contact, but as I was walking away I heard them giggling as a group. I’ve heard that girls giggle when they are nervous, but I am also a bit self conscious and wanted to know what y’all think. I don’t consider myself to be hideous, but I genuinely don’t know what they were laughing at.
Girls, Why do girls giggle?
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