Carbonated water, the water-sugar solution used for making rock candy and rain clouds are all examples of supersaturated solutions. Supersaturated solutions consist of an over-abundance of solvents in liquids or gases compared to what can be dissolved within normal conditions.
When some liquids are heated or cooled, larger amounts of solvents can be dissolved. However when the temperature returns to normal, excess solvents crystallize and precipitate downward in the solution.
A closed container of completely purified water cooled below the freezing point can stay in liquid form if no mineral or gas particles are introduced. The water is supersaturated with dissolved ice crystals, but it is the introduction of additional particles that triggers the conversion to crystal form