If you add water to Magnesium Chloride Hexahydrate (a powder at room temperature), you get more water than you had before... This will work with any hydrated salt...
For instance, dissolve 150 grams of fully hydrated magnesium chloride in 100mL of water, and you will end up with 185mL of water.
http://www.google.com/patents/US4008170
http://www.nature.com/news/2008/080903/full/news.2008.1077.h...