Hard water
Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca and Mg. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Postage stamp paper
Another comparative set of terms refers to the paper's hardness. "Hard", "stout hard", and "soft" have been used to describe the paper. Experts have described the "snap" of the stamp when "flicked" as a means to determine if the stamp was printed on hard or soft paper. A sharper snap implied hard paper because the hardness is a characteristic of the amount or kind of sizing used when making the paper.