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Surface tension is a property of liquids that arises due to the imbalance of cohesive forces at the surface of the liquid. Water molecules at the surface experience inward cohesive forces which cause the surface of the water to contract, minimizing its surface area. This is why water droplets form into spherical shapes – a sphere has the smallest surface area for a fixed volume.

Wax (car wax specifically) is hydrophobic, which means that it repels water. When the car's surface is coated with wax, the wax molecules form a thin layer that interferes with the ability of water molecules to bond with the surface of the car. This leads the water molecules to bond more strongly with each other rather than the surface.

When water comes into contact with the waxed surface, the surface tension of the water and the hydrophobic nature of the wax work together to create water beads. Instead of spreading out on the surface, the water molecules prefer to remain close to other water molecules, forming tight clusters or beads, minimizing contact with the hydrophobic wax.

Water beading can be beneficial as it prevents water from creating larger pools on the surface, which could lead to potential damage over time, such as rust and corrosion. Additionally, the round shape of the beads makes it easier for them to roll off the surface, taking dirt and contaminants with them, which provides further protection to the car's paint. In conclusion, water forms beads on a waxed car finish due to the surface tension of the water and the hydrophobic nature of the wax. The water molecules are more attracted to each other than the wax surface, and the surface tension causes them to form the smallest possible surface area – which results in beads. These beads can provide additional protection to the car's paint by aiding in the removal of dirt and contaminants.