Oxygen is one of the most common elements in living things, the Earth, and the entire universe. One of the most important characteristics of oxygen is that it is highly "electronegative" which means that it has a tendency to attract electrons. As a result, it can react with nearly all elements in the universe to make "oxides" which are chemical compounds that contain oxygen. Oxygen is a major component of air and water. Many living things undergo "respiration" in which oxygen is necessary for the organism to create the large amounts of energy needed to sustain life. The importance of oxygen in respiration is due to its electronegativity since at the end of respiration it combines with hydrogen to form water. Oxygen is also the waste product of plants during photosynthesis where they combine sunlight, CO₂, and water to make sugars.