I was at my sister's house and her 4-year-old granddaughter asked me that question. It sounds simple at first but try coming up with an answer.
I'm not an expert but water acts as a base or acid depending, it expands when it freezes (pretty unique) and it's pretty important to a lot of the chemistry (life) that happens on our planet. Tell her that, and she can find the rest online lol. My nephew recently asked me if I knew what l.e.d. stood for. I don't have kids, so I wasn't sure if I should come clean (he was excited to have learned it), but I did and he was happy to confirm that I was "absolutely correct". Kids are pretty smart these days.
I was going to say glue. Seems everything organic is mostly held together with water. OR mostly water. Only substance I know of that can be in three states in one location at the same time.