At any rate... if the scientists can make a scale that measures how much water you drink, then I think they should account for something else. What if something like this happens?
Say you're weighing yourself on the scale and all of a sudden you have to go number two... so you just go while you're standing on the scale. Would the scale be able to tell that was happening? Probably, if all of it landed on the scale. Maybe they could program the scale to say something clever like, "Impressive" or "Nice one, sir."
Now how about this: What if not all of the poop lands on the scale? Then you'd have a different weight before and after you stepped on the scale. I'll bet they could make a scale that could tell when that happens. Maybe then the scale could say something really wacky, like a sarcastic, "Nice aim, Captain Bullseye."
My apologies to Jules Verne if an idea like this has peeked it's turtle-like head out of its shell before... otherwise,