Missing here is a critical part of the analogy - if it's to apply to the internet, we have to assume that the contaminants we are speaking of are put back INTO the system from the end user, just just delivered in one direction. Rare, I would assume, is the ability of a water end user to put back water into the system, unless we also speak of the waste disposal system too 
/john
On Wed, Apr 27, 2005 at 03:19:04AM -0700, Owen DeLong is reputed to have mumbled: