Cohesive forces are responsible for surface tension, the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water molecules at the surface (at the water-air interface) will form hydrogen bonds with their neighbors, just like water molecules deeper within the liquid. However, because they are exposed to air on one side, they will have fewer neighboring water molecules to bond with, and will form stronger bonds with the neighbors they do have. Surface tension causes water to form spherical droplets and allows it to support small objects, like a scrap of paper or a needle, if they are placed carefully on its surface. Water likes to stick to itself, but under certain circumstances, it actually prefers to stick to other type of molecules. Adhesion is the attraction of molecules of a different kind, it can be quite strong for water especially, with other molecules bearing positive or negative charges.

A water molecule is formed by a combination of two hydrogen atoms and one oxygen atom. The hydrogen atom bonds to each oxygen atom with a pair of shared electrons. An oxygen atom also has two additional pairs of electrons. Overall, a water molecule has a neutral charge. However, because of the unequal arrangement of its electrons, the oxygen end is negative, and the hydrogen end is positive. Opposite charges are attracted to each other, so when there are lots of water molecules, they stick together.

These attractions are an example of hydrogen bonds, weak interactions that form between a hydrogen with a partial positive charge and a more electronegative atom, such as oxygen. The hydrogen atoms involved in hydrogen bonding must be attached to electronegative atoms, such as O\text{O}OO, N\text{N}NN, or F\text{F}FF. Water molecules are also attracted to other polar molecules and to ions. A charged or polar substance that interacts with and dissolves in water is said to be hydrophilic: hydro means "water," and philic means "loving." In contrast, nonpolar molecules like oils and fats do not interact well with water. They separate from it rather than dissolve in it and are called hydrophobic: phobic means "fearing." You may have noticed this as a not-so-handy feature of oil and vinegar salad dressings. Vinegar is basically just water with a bit of acid.