Edwin Cartlidge on New Scientist wrote a great piece about the mysteries of water
We are confronted by many mysteries, from the nature of dark matter and the origin of the universe to the quest for a theory of everything. These are all puzzles on the grand scale, but you can observe another enduring mystery of the physical world - equally perplexing, if not quite so grand - from the comfort of your kitchen. Simply fill a tall glass with chilled water, throw in
an ice cube and leave it to stand.The fact that the ice cube floats is the first oddity. And the mystery deepens if you take a thermometer and measure the temperature of the water at various depths. At the top, near the ice cube, you'll find it to be around 0 °C, but at the bottom it should be about 4 °C.
That's because water is denser at 4°C than it is at any other temperature - another strange trait that sets it apart from other liquids.
Water's odd properties don't stop there...