A paper towel absorbs water due to capillary action that carries water droplets into the voids in its structure. When a paper towel comes in contact with water, the water moves along its fibers, spreading throughout the towel. Once the water enters the gaps between the paper fibers, the surface tension keeps it trapped. Thicker towels have more gaps and can therefore absorb and hold more water.
The reason water moves into a paper towel's fiber voids is that its molecules are polar. Each end of the molecule holds a distinct electrical charge, which means that water is attracted to a wide variety of different materials. In a paper towel, the water molecules are attracted to the paper fibers. This attraction, combined with surface tension, can even allow water molecules to move against the force of gravity, climbing upward when a paper towel is dipped into a water source.
Thin paper towels are not initially as effective at absorbing water, but folding one in half can significantly increase its absorbent capacity. Doubling or quadrupling a paper towel creates additional air gaps in its structure. This produces even more pockets for water to be drawn into and held by surface tension.