Assume the molecules of water to be cubes in close contact (the volume of water is completely filled by the cubes). Determine the number of molecules that fit in V' = 1 nm³ of volume. Consider the water to be at standard conditions (T ≈ 300 K, p ≈ 1 atm), look up the necessary values of water density, molar mass and Avogadro's number (if you don't know them by heart). Mirek didn't see the point of including such primitive problems in master's studies.