Chemical reactions can occur spontaneously if the reactants possess more potential energy (stored energy) than the products. This type of reaction occurs spontaneously because of the downhill energy path (from more potential energy to less). These reactions are called exothermic (heat-producing) reactions, because potential energy is converted to heat as the reactions proceed. Conversely, endothermic (heat-absorbing) reactions do not occur spontaneously because of the uphill energy path that exists. The products of endothermic reactions contain more potential energy than the reactants. As a result, energy must be added to trigger an endothermic reaction.