How does hydrogen bonding work?

Hydrogen bonding is a relatively strong inter-molecular force, caused by strong dipole-dipole interactions between an electron-deficient H atom and a lone pair of electrons on a highly electronegative atom on another molecule. Key electronegative elements to remember are Fluorine, Oxygen and Nitrogen (FON), as these are all highly electronegative and therefore when bonded to hydrogen (e.g. HF, H₂O and NH₃) form strong dipoles which give rise to hydrogen bonding.The phenomenon of hydrogen bonding is what gives rise to many of the seemingly anomalous properties of water, such as its relatively high boiling point relative to other hydride molecules - this is because the relatively strong hydrogen bonds must be overcome in order for evaporation to take place. Similarly, when freezing, liquid water becomes most dense at 4 degrees and as it freezes actually becomes less dense as its hydrogen bonds form a crystalline structure.