The answer is;
- from solutions that evaporate
- from hot water solutions when the solution cools
The solution has to be saturated so as to be able to form crystals. Evaporating the water makes the remaining solution concentrated t saturation. Also, heating allows the solution to dissolve more solute and when it cools, the solution becomes oversaturated and crystals begin to form. For larger crystals to be formed, the solution needs to cool or evaporate slowly and the solution needs to be very still.