And one example of a spontaneous reaction in nature is combustion. So combustion reactions are examples of exergonic reactions where the Delta G value is negative. Now, what about the opposite? Well, if we read this reaction going backwards, if this is the reactant and this is the product that if we subtract a high free energy from a low free energy, we're going to get a positive Delta G. And a positive Delta G means the reaction is endergonic and non spontaneous. And that means it will not take place unless we input a certain amount of energy. And one example of an endorganic reaction that is not spontaneous is the synthesis of ATP molecules inside our body.