According to the United States Geological Survey, capillary action occurs due to the forces of surface tension, cohesion and adhesion. Capillary action is the tendency for water to move with the spaces of a porous material. For example, when a paper towel is placed in a glass of water, the water travels up the paper towel until the force of gravity overpowers the capillary forces.
Cohesion is the tendency for water molecules to stick together, whereas adhesion is the tendency for water molecules to stick to other substances. When combined with the tendency of water to resist external forces, which is also known as surface tension, water and other liquids climb up the sides of a thin glass vessel, called a capillary tube. HowStuffWorks explains that the diameter of the capillary tube and the ambient temperature affect the distance that the liquid rises.
There are many examples of capillary action in the natural world. According to HowStuffWorks, compact soil contains small spaces between the grains of dirt. Water rises through the soil via capillary action and evaporates upon reaching the surface. Additionally, as explained by USGS, plant roots rely on capillary action to draw water from the ground and transport it to the plant’s trunk, branches, twigs and leaves.