Hydrogen bonding is a type of weak chemical bond that occurs when two electronegative atoms, such as oxygen and nitrogen, interact with the same hydrogen. This is also known as a strong form of intermolecular interaction. Hydrogen bonding is a strong form of intermolecular attraction. There are many elements that will combine with hydrogen to form compounds, in this case, also known as "hybrides". Under physiological conditions, water molecules rarely ionize to form H+ and OH- ions, instead, they form strong hydrogen bonds. This hydrogen bonds, which are constantly being broken and reformed in water, have approximately 1/10 the strength of an average covalent bond. Every water molecule can potentially form four hydrogen bonds with the help of its surrounding water molecules. 
Hydrogen bonds between base-pair in DNA
- ↑ Watson, James D.; Baker, Tania A.; Bell, Stephen P.; Gann, Alexander; Levine, Michael; Losick, Richard (2004). Molecular Biology of the Gene (5th ed.). San Francisco, CA: Pearson, Benjamin Cummings. p. 49. ISBN 0-8053-4635-X.