When you take a bath, you are soaking off all of the dirt that has accumulated on your body and in your pores through the course of the day. All of your loose skin cells drift off. You now have dirt and dust particles in your bath, and because the densities of these particles are much less than water, they float. Then you add soap into the mix, and when the suds and soapy residue are rinsed off of you, they too float. When you drain your tub, the particles on the edges will adhere to the side of your tub, thus creating the dreaded ring-around-the-tub.