If you look at the roof and any house you will see short black pipes sticking up. They are not even a foot tall and maybe just a few inches in diameter. Depending on the size of the house there are anywhere from one to, well...lots. Those black pipes, if you were able to follow them down through the roof, down the walls, are connected to your bathroom fans. Some houses have more, for instance we have a fan in our laundry room and kitchen. Anyways, I learned all this out the hard way.
Back when I used to make a living remodeling houses I ended up spending much time on the roof of this one house. Well, one day when I was up there, my co-workers and I heard a toilet flush and sure enough, seconds later, standing next to that pipe smelled like changing a diaper.
This same house was five doors down from my parents house, where I was living at the time. Same street and everything, just five houses down the rode. Everyday for work I would only need to leave my house five minutes before my shift started. It was quite nice, but the interesting part was that I drove straight there and parked, then on my lunch break and again on my way home, I just kicked it into reverse and backed all the way home. I honestly did this every single day, twice a day, five days a week for about three months. If the neighbors looked out their windows, at almost exactly the same time everyday, twice a day, they would see a small green truck driving in reverse down the street.