Smallest Molecules for Water to be Fluid: Why?

[Lorentz]

This was a national science quiz question:

What's the smallest amount of molecules for water to have the properties of a fluid.
a)1 molecule
b)2 molecules
c)6 molecules

What's the answer and why?

 

[Gokul43201]

The best answer is probably (c) 6 molecules, but really, you may need a lot more. (a) is ruled out from viscosity. If you want a liquid, you must see surface tension, which requires at least a bilayer. You can't make a stable bilayer with only 2 molecules. You might also want to have indistinguishability. There are questions and complications arising out of the method used to contain these molecules.