How do we know that any life form needs water? How can we be sure that there isn’t some crazy life form on another

Question: How do we know that any life form needs water? How can we be sure that there isn't some crazy life form on another planet that's completely different to any organisms on earth, and doesn't need water to survive?
- Keywords:
  life,
  planets,
  water
Asked by javanlangar to Claire, Sam on 28 Jun 2013.
- Claire Lee answered on 28 Jun 2013:
  
  I’m sure there are other life forms like that! Life on earth developed to need water because there was lots of water available, so that kind of made sense. Or, more likely, perhaps some creatures who didn’t need water but needed sulphuric acid to survive instead also developed, but since there wasn’t much sulphuric acid around they didn’t get very far down the evolutionary chain.
  
  In fact, we can almost be sure that if we find life on a different planet it will likely be completely different to us, rather than the same, because of the different conditions that it evolved under!
- Sam Geen answered on 28 Jun 2013:
  
  We can’t be. It’s difficult to say how you would make life that is chemically different from life on Earth because you’d have to come up with something like DNA, etc. There’s a fairly good discussion about why water is important here: http://science.howstuffworks.com/environmental/earth/geophysics/water-vital-to-life.htm

Question: How do we know that any life form needs water? How can we be sure that there isn't some crazy life form on another planet that's completely different to any organisms on earth, and doesn't need water to survive?