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In a water molecule the electron shell round a hydrogen atom is rather thin, and the positive charge on its nucleus shows through to the outside world, giving the hydrogen atom a small positive charge. On the other hand the electron shell round an oxygen atom is quite thick, and so oxygen carries an extra bit of negative charge. These opposite charges attract, although quite weakly.

This weak force is called a hydrogen bond. The hydrogen atoms of one water molecule stick to the oxygen atoms of nearby water molecules, freezing them together into solid ice.

Hydrogen bonds occur in other molecules as well as water. Ammonia molecules can also stick together when they are cold. Hydrogen bonding can only happen in molecules that have a permenent dipole (as water does) and that also contain the highly electronegative elements fluorine, oxygen, or nitrogen.

Not all molecules are sticky. Those which cannot form hydrogen bonds we can think of as slippery. The scientific name for this is "hydrophobic" which means "water fearing". Examples of slippery molecules are fats and oils. On the other hand molecules which stick to water, such as alcohol and sugar, are called "hydrophilic", meaning "water loving".

The difference between slippery and sticky molecules is very important in the process of life. For example the folding of amino acid chains into enzymes relies upon it. So too does the holding together of cell membranes.

Not all stickness of molecules is due to hydrogen bonding. There are very weak bonds, called van der Waals bonds, which occur between nearby molecules because of random fluctuations in their electron shells.