Essentially, the passing around of this idea is like a big game of telephone. Do you remember that game when you were in 5th grade? Everyone sits in a circle or line. The person who starts whispers something in the ear of the person next to them. That person in turn whispers the same statement to the next person. When it gets to the end, you see how different the statement has become.
Well, obviously, people need some amount of liquid. That little starting point has morphed through word of mouth over many years into
you have to drink eight glasses of water per dayand
caffeinated drinks don't countamong other things. Is it true? It could be, but no one has ever checked. The evidence cited in the paper suggests that it is not.
Now think of all the other crap you've been told over the years, and all the crap that you've passed on yourself. Legal statements are a good place to start. Or what you've been told about the internet and how it works (particularly viruses). There are a number floating around about health. And I'm not counting all the urban legends about some axe-wielding marauder who hides in the back seats of cars either.