The chemical symbol for water can be written as H2O, with the 2 written is subscript, or as HOH. HOH is the less-common symbol used when referring to water.
Water is written as H2O or HOH because every molecule of water contains two hydrogen atoms and one oxygen atom. These three atoms are bonded together through covalent bonding. Covalent bonding allows for these atoms to share their elections, enabling fluidity. The opposite of covalent bonding is ionic bonding, which does not allow for transferring of electrons. Water is the only substance that exists naturally as a solid, liquid and gas.