Why does my friend do this?

so, i have this friend, lets call her Girl. So, Girl has been getting on my nerves. for example, if i say something like, "there is such a think as Milkis." (milkis is actually a drink but Girl disagrees cuz she hasnt seen it b4 cuz its rare.) then she says REALLY loudly so the whole cl can hear, "Milkis doesnt exist!" or sometimes she will add in a (lets say my name is Margot) "You're so stupid Margot". then she acts like i owe her one of those drinks wen i only suggested i could maybe bring one in so she could see. Why does she do this and how should i deal with her rudeness? thanks guys it means a lot :)