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.