forming the "hydrogen bond"... is dynamic, with the exchange occurring in a few billionths of a billionth of a second. Since water molecules can "lose" two hydrogen atoms and "win" two by way of their oxygen atom, they are on average regrouped in fours, and the network of water, whether in our glass or in our cells is tetrahedral. All the same, this arrangement is not perfect in liquid water (otherwise the water would be a solid crystal), and locally there are many defects in the network. Although the motif of four water molecules is the most common in "linear" hydrogen bonds, the water molecule sometimes has a "forked tongue" with bifurcated liaisons, which creates arrangements of three or five, or even two or six molecules. This variability prevents water from being structured over a great distance and enables it to remain in a liquid state at the ambient temperature.
Denis Le Bihan
water