Water is a polar molecule. Each water molecule has a slightly positive end and a slightly negative end — like a tiny magnet. Paper towel fibres are made of cellulose, which is also polar. Opposite electrical charges attract each other: the water molecules are pulled toward the cellulose fibres. This attraction between unlike molecules is called adhesion.

Once the first water molecules move up into the fibres, they don't go alone. Water molecules are also attracted to each other — a force called cohesion. They pull each other along in a chain, the way a string of paper clips moves when you lift the first one. 

Together, adhesion and cohesion create capillary action — a climbing force strong enough to beat gravity over short distances.

But gravity doesn't disappear. As the water climbs higher, gravity pulls back harder. The apex of the paper towel arch is the turning point — the highest point the capillary force can reach before gravity wins. From there, gravity assists, and the water drips down into the empty cup.

The experiment reaches equilibrium when all three cups are at the same water level. At that point, gravity and the capillary force are exactly balanced, and flow stops.

This is not a party trick. It is the mechanism that keeps every forest on Earth alive. The xylem vessels inside a tree trunk are extraordinarily narrow tubes — far narrower than your paper towel fibres. Water molecules are pulled upward through adhesion to the vessel walls and cohesion to each other, in an unbroken chain from the soil all the way to the topmost leaves. In the world's tallest coast redwoods — over 115 metres high — that chain is working against over 11 atmospheres of gravity. 

It works because the tubes are narrow enough, and the molecular forces are strong enough.

Curiosity spark: Capillary action works because the passages are narrow. What do you predict would happen if you used a wider material — like a sponge with large pores instead of paper towel with tiny fibres? Would the water climb higher, lower, or not at all?