Culture: Females, tell me what this means?

A female giggles when I say something to them. I never met the female in my life but I say something and they sit there and giggle or in some cases just bust out laughing.. I ask what's so funny and they reply' "nothing", then they proceed to talk to me normally. Guys never do this...this is bi-polar behavior coming from females that I do not understand. Explain this to me..Males can also chime in.