When very thin cylindrical tubes, called capillaries, are dipped in a liquid, the liquid rises or falls in the tube compared to the surrounding liquid. This phenomenon is called capillary action. Capillary action occurs due to the combination of two opposing forces: the cohesive forces of the liquid, which cause it to stick to itself and form a rounded shape, and the adhesive forces between the liquid and the walls of the container, which cause the liquid to be attracted to the container walls.

An example of capillary action can be seen in water rising in a narrow glass tube. The cohesive forces of the water molecules cause the water to form a rounded shape at the top of the tube rather than flowing down the sides.

Another example can be seen in plants, where capillary action is responsible for the transportation of water and nutrients from the roots to the different parts of the plant. The narrow tubes in the plant's stem, called the xylem, rely on capillary action to move water upward against the force of gravity.

Capillary action also plays a role in many industrial processes, such as papermaking, dyeing, and chromatography.