No one is entirely sure. Water has high surface tension, yet the molecules at its surface are unstable, which makes the air-surface interface mysterious and difficult to describe. Satoshi Nihonyanagi, a researcher at a molecular-spectroscopy lab in Japan, studies water and its surface—"specifically probing the interface," he calls it—using isotopically diluted H2O, which doesn't vibrate and is easier to observe. He's found that some water molecules are bound to others by a single hydrogen bond. "Occasionally," he says, "this hydrogen bond can break, and the water at the surface escapes into the air." Hydrogen's breaking and re-forming happens trillions of times a second. The hydrogen atoms "point" to their neighbors, inching toward them. But water molecules don't pair up for long. They are swingers, constantly coupling and uncoupling.