Why do girls a lot of times laugh a little when a guy compliments them on their appearance?

I have noticed from just spectating guys on YouTube "picking up" girls and also in real life when I am trying to start a conversation and talk to my crush, when I compliment them, even like on a piece of clothing, a lot of times that will sort of break the ice and they will go from completely serious and indifferent to laughing/blushing.
Why do girls a lot of times laugh a little when a guy compliments them on their appearance?
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