Since your shit can be approximated with "particle in a 3-dimensional box", I would suggest Schrödinger's equation.
In the bottom of your toilet, the potential rises for the wave-function. but unlike the walls of your can, this is a mere potential step, not an infinite potential wall. (by this is meant that no matter how hard you push, you can't shoot a one that would pierce the wall). in the other hand, in the bottom there is only a finite rise in potential.
After the fall, the total energy of the turd is lower than the potential of the step because it has hit the water. So classically speaking, the turd stays in the can. In quantum mechanics, however, there is a phenomenon called "turd-tunneling"
(I only found a picture of electron-tunneling which was later derived from turd-tunneling)
I would think that you this is the case here. After working hard, you blast out a piece that would make anyone proud, but alas, it is gone with the drains before you get a chance to call your girlfriend to see. When you look into the can, you only see a little piece that was, in fact, already hanging from your ass hair when you first walked into the bathroom. I think I have heard someone call this "self-esteem killer" or something like that.
sorry, gotta stop now. I have a terrible hang-over and I stink so much it is making me nauseous...
