Where does the toilet paper go?

This is the kind of thing that bothers me on two levels of green. Financially and environmentally.

I have no idea how only two girls can go through as much toilet paper as we seem to in this house. I'm pretty aware of it, and am confident that I am not over-using to any degree, yet I bought a 12 pack less than two weeks ago, and my roommate just bought another the other day. We currently have 16 rolls, I plan on watching to see how quickly they disappear.

I personally believe that 16 rolls should last us about a month at least, but it seems as though they will be closer to two and a half weeks. I don't know that there's anything to be done if my roommate is using it up way too fast (how do you tell a grown woman that she's using too much toilet paper?) but I can try to entertain myself by noticing the rate of disappearance.