But water can also dissolve some nonpolar molecules, such as ethane, by forming cages of water molecules held together by hydrogen bonds. If the concentration of such a nonpolar molecule gets too high, the nonpolar guys will all get together in a sort of molecular wagon train circle to minimize their exposure to the water—but that’s important to living systems too, as we’ll see in our next exciting episode. The point is that water is an extraordinarily versatile solvent because of its molecular structure.

What’s more, water is abundant in the universe. This is a key feature of substances found in living systems, and that’s why scientists get so flushed and trembly when they find a world with water on it, like Europa or Callisto. There are other thallasogens (sea-forming substances, a word coined by Asimov) that could conceivably support life elsewhere in the universe, but none of them combine the cosmic abundance and unique physical properties that would make them as desirable as good old H₂O. Ammonia, for example, is an excellent polar solvent, and quite abundant, but it’s only liquid at very low temperatures. And when it freezes, it sinks.