Self-heating systems for rapidly and effectively heating a comestible substance are disclosed. Self-heating systems generally include a reaction chamber and a heating chamber. The heating chamber contains a substance to be heated. The reaction chamber contains reactants that, when contacted, exothermically react. The containers and reactants can be configured to heat at least six fluid ounces of comestible substance in less than one minute. The solid chemical reactant mixture can comprise magnesium chloride, calcium chloride, and/or calcium oxide. Methods for heating at least six fluid ounces of comestible substance in less than one minute are also provided.