why do paper towels soak up water

Paper towels are made up of cellulose fibers, which also make up cotton, wood, and most other plants. These cellulose fibers are actually giant molecules that consist of many small molecules linked together. The small molecules that combine to make up cellulose are sugar molecules; that s the key to the absorbency of paper towels.

Think how easily sugar dissolves in water. When you get a paper towel wet, the water molecules rush in and cling to the cellulose fibers. That s why paper towels are great at picking up spills. Although cellulose contains sugar molecules, that doesn t mean paper towels are edible.

We humans don t have any of the enzymes necessary to split the cellulose molecule apart into the individual sugar molecules. That s why paper towels have no nutritional value for us.
Capillarity, or capillary action, occurs when intermolecular bonding between water molecules and molecules of another material is greater than the intermolecular bonding among water molecules themselves.

This is called adhesion, and when it happens water will tend to spread over the surfaces of the other material, even climbing against gravity to do so.

Cohesion among the water molecules will drag along those water molecules not in direct contact with the material, and surface tension will hold the water together as it adheres to the surface. Adhesion may be strong enough to raise the water against gravity.

If a material is porous enough, the water can continue to climb for quite a distance. But on a smooth non-porous surface, the water will stop climbing when the weight of the cohered water is greater than the adhesion can support.