I'm not really sure where to put this, but I found this to be an intriguing engineering question along of the lines of hydrodynamics. However, lacking any particular category devoted to that field... Here's the issue: I spent the past four years in Germany. When I flushed my toilets, they flushed, completely, and without using tons of water - zero reamining residue. I settle in my apartment in the US, and when I flush, a large amount of residue remains. But here's the curious thing: I flush again and it's all gone! I'm thinking if each flush only removes 60% of the residue, by the second flush, only 16% will remain. But in reality, none remains. Why is that?