Fractional freezing is a process used by chemists to separate two liquids which have a different freezing point. This happens by dropping the temperature of both liquids - one liquid will reaching its freezing point and solidify, whilst the other will remain liquid. This liquid can be poured off, leaving the solid to warm and return to its liquid state.
This process is used to increase the alcohol concentration in hard cider to make Applejack. It is also used in ice beer.