Ugly Bags of Mostly Water

Coworker asked why I have a bag of water on my desk, and I responded with "you wonder why I have an ugly bag of mostly water?" and got a satisfying guffaw from Joe in the cube next to me. Kirk, the not so geekly guy in the other cube who asked the question in the first place, didn't get it and we explained it was a classic description of human beings but couldn;t agree where it comes from. Looking on the google I sound a refrence to an ST:TNG episode but I was sure I had seen it before then, so was Joe. He thought it was classic trek, I was thinking it was something I read in the 80's like Hitchhikers or Niven.

Anyone have any better references?