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.
Grit (personality trait)
Salvatore Maddi (2006) defined "hardiness" as a combination of attitudes that provide the courage and motivation to do the hard, strategic work of turning stressful circumstances from potential disasters into growth opportunities. While grit is primarily a measure of an individual's ability to persist in obtaining a specific goal over an extended time period (Duckworth et al., 2007), hardiness refers to an individual's ability to persist through difficult circumstances and does not address the individual's long term persistence toward a specific goal. Maddi (2006) developed a theoretical model of hardiness as a tool for developing resilience.