Any high-sucrose syrup will crystallize under the right conditions. Temperature, agitation, saturation level and condensation all factor into crystallization, or the formation of crystals in a solution. Molasses, the syrupy byproduct of sugar refinement, is primarily sucrose, but the concentration varies depending on the source -- usually sugarcane, sugar beets or sorghum. Generally, the more sucrose the molasses contains, the more likely it will crystallize. However, just as easy as molasses crystallizes, you can just as easily de-crystallize it, too, and make it less likely to happen again by adding citric acid or pure fructose to it after softening it.
Open the jar of crystallized molasses and place it in the center of a saucepan or pot.
Add enough water to the saucepan to reach the level of the molasses in the jar.
Heat the saucepan on the stove over high heat until the water boils.
Boil the water until the molasses melts.
Stir in a tablespoon of corn syrup or water, or 1/2 teaspoon of lemon juice using a metal or silicone-coated utensil while the molasses is hot to prevent it from crystallizing again.
Turn the heat off. Remove the jar from the water and let the molasses cool to room temperature. Store molasses in the freezer or at room temperature.