Nitrogen can be fixed by lightning converting nitrogen gas () and oxygen gas () in the atmosphere into  (nitrogen oxides). The  molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms. Lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming . These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form , which in turn reacts with water to produce  (nitrous acid) or  (nitric acid). When these acids seep into the soil, they make  (nitrate), which is of use to plants.