Why is it?

Why is it that when a girl borrows a pen to write with for one class, she never returs it? I have "loaned" out at least 8 pens this semester alone, about 3 to guys, that I have got back, and the other 5 to girls, whom of coarse never returned them. This is yet another huge difference in men and women. Men returned loaned goods, while women do not.