I have been reluctant to ask this question on this website because it sounds so ridiculous, but as Hillel said in Avot 2:6 "one who is bashful cannot learn" so here goes...
Why is it permissible to flush a toilet on Shabbat?
This question asks it from the point of view of water pressure and causing pumps to turn on, but I do not see why it is not moving material (human waste) from rishut hayachid (a private domain) to either rishut harabim (public space) or karmelit (river or similar place where public does not go) (definitions of terminology here).
I read this idea years ago on a humorous "How to mess with Baal Teshuvas" list, but it stuck in my head because I really did not have an answer for such an thing. I am sure there is solid halachic reasoning on why it is perfectly permissible, but I am just not learned enough to know what it is. Please clue me in!