Our study of life begins with an examination of chemistry because all living organisms are composed of chemicals, and all life processes follow chemical rules. All matter, living and nonliving, is made up of atoms. Atoms can interact by transferring electrons or by sharing electrons and forming molecules. Atoms and the bonds that hold them together contain energy; this energy is harnessed and utilized by the chemistry of life. The single most important molecule for life as we know it is water. Water's unique and vital properties are a direct consequence of its chemistry.