What is it with girls?

Today we had a snow (more like freezing rain, icky weather) day. So I had my son, daughter and my friend's daughter here. My daughter is 9 and my friend's daughter is 5 and usually the pair get along, however today they were like pitbulls in a dog fight!

I'm still getting use to two girls together I guess. I never had a sister growing up, so I missed a lot of that, and now my sis-in-laws and I get along fine, so we don't fight at all. These two girls today however had to argue over the dumbest things from paint getting onto clothes to who got to sit on what side of the sofa and in what position. It was absurd that two intelligent beings could find the most ridiculous subjects to argue about.

I had to turn into the evil Mom. Not something I like doing in front of any one but my own kids who have grown up to it and are well used to my dropped voice bellow. In this case, the words were simple - "SIT. WATCH THIS MOVIE. DO NOT MOVE, LOOK AT EACH OTHER OR TALK TO EACH OTHER OR TOUCH EACH OTHER." It felt a little more like commanding a dog than disciplining children. Maybe there is not that wide a difference between the two?

I still can't understand though--what makes females so much more evil when it comes to getting along. If my son has issues with a friend, he'll walk away and ignore. These two girls, however, took more to deliberately trying to ignore each other and see who would cry "Mom/Tracy" first. I know as we are older hormones play a big part of this but at 9 and 5 there are not the hormonal mood swings yet? Are there????