Compounds are substances made up of two or more different elements that are chemically bonded together. They are pure substances and are formed when atoms of different elements react or when electrons are shared or exchanged between atoms. Examples of compounds include water, which is made of hydrogen and oxygen, and carbon dioxide, which is used in photosynthesis to create oxygen through sunlight. Ethanol, an organic compound, is used as a renewable fuel source, often blended with gasoline to reduce emissions, and as a solvent in cosmetic products and pharmaceuticals. These compounds are essential for maintaining the body and promoting the production of oxygen.