Water is a molecule that contains two atoms of hydrogen and one atom of oxygen. It can exist as a solid, liquid or gas on Earth, depending on the temperature. It covers 70.9 percent of Earth's surface. Living things depend on water to survive. The oxygen atom has a slight negative charge relative to the two hydrogens atoms which are slightly positive, making water a polar molecule. There are strong attractive forces between water molecules. This accounts for capillary action, when water molecules can move up a narrow tube, against the force of gravity, allowing it to travel upwards from roots, to stems to leaves, in the xylem tubes of conducting tissue in plants. It is a universal solvent and important substances for living things such as salt, sugar, acid, alkalis and gases as well as protein, DNA and polysaccharides, can dissolve in it.