Why do girls giggle when they see me?

I've had this happen 3 times so I'm sure it's something recurrent

its like as soon as they finished looking at me they look down and giggle at themselves. I'm 18 so the girls aren't super young, mainly 18-20 years in Campus who do that. How do I know for sure if it's because they're attracted to me or they think I'm a joke
Why do girls giggle when they see me?
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