How many water molecules does it take to make the smallest possible ice crystal?
Alright, I've not actually been dying to know this, but, the explanation of the answer is pretty interesting none-the-less.
I can understand why 1 water molecule won't freeze since ice is after all a collection of water molecules rearranged in a different lattice pattern. But you'd think that 2 molecules of water could freeze easier than 50, or a million, or the countless millions that would make up an ice cube. Well, you'd be wrong if you thought that way.
Turns out the freezing process is related to the vibrational frequencies of both hydrogen and oxygen bonds, and that the intermolecular vibrational frequency of the hydrogen bonds are what hold water molecules together. Science has known for some time that 50 molecules or less can't seem to stay together as an ice crystal, but counting and watching more than 50 molecules was a serious limitation, so they didn't really know how many molecules was the critical number.
The answer came when a process was designed to organize and count more than 50 water molecule clusters at a time. Researchers at the Institute fur Physikalische Chemie (Gottingen, Germany) found a way to do this, and so the trials began. It turns out that 275 water molecules is pretty close to the magic number - any fewer and ice crystals don't form. Moreover, the freezing begins at the center of the 275-molecule cluster where a ring of six hydrogen-bonded water molecules forms in a tetrahedral configuration - the smallest ice crystal possible.
What good is this information one might ask? It is expected to aid in climate modelling now that science better understands the extent and timing of the process of ice crystal formation from atmospheric water. Knowing better how this process influences cloud formation will also give us more information on the earth's radiation budget. Besides all that, it's just a piece of trivia that should be in every water-wonk's data base.