Where sea meets sky, there are lots of water molecules with an identity crisis. About a quarter of the H2O in water’s uppermost layer can’t decide whether to be liquid or gas: One hydrogen atom stays in the drink while the other pokes up, vibrating in the air. This layer of molecular ambiguity is extremely thin and has little or no effect on the water below it, new data reported June 9 in Nature show. Right beneath the liquid’s surface, water molecules go about their business just as if the air weren’t there. That may seem like a dull discovery,...