Water, out of any compound on Earth, or even in the universe itself, has some of the most unique physical properties. Unlike most materials, when water turns from a liquid to a solid, it expands. This quality is what allows ice to float at the top of water. If ice did not float, all the water on earth would eventually freeze, leaving humanity to die in a frozen wasteland.
This property is possible due to the way water molecules interact with each other. When in its liquid state, water molecules are much closer together. They slosh together and attract each other in much more fluid water than other compounds. This trait is due to something called hydrogen bonding. Essentially, the single oxygen atom and the two hydrogen atoms are electrically charged in such a way that each water molecule is able to connect very closely and fluidly to the ones around it. This bond is rather weak when compared to most bonds, hence water’s ability to move in such a liquid manner. This property also causes surface tension, which is why a bit of effort is needed to pierce the surface of water. The electrical charge of water also allows it to stick to many surfaces. This is why streams of water can sometimes be seen running upside down along ceilings. The combination of hydrogen bonding plus the water molecules bonding to the roof keeps them there as they flow along.
Hydrogen bonding allows for many things. Without it, as well as the properties of osmosis and pressure, trees could not transport water up to their highest extremities. Hydrogen bonding is also responsible for the condensation of water droplets. Even the droplets on the side of a freshly waxed, smooth car. This attribute is accentuated because wax has no charge. It is neutral. So, when rain falls or water is sprayed onto the car, the water molecules have nothing to bond to except themselves, causing even tighter, bigger droplets of water to form.
Conversely, if a car is not waxed, the surface is not as smooth, leading to water droplets not condensing as well and sliding down the car in sheets. What does not smooth mean? Essentially, this means that the car has a lot of excess, stray material ‘sticking out’ of its surface on a microscopic level. These protrusions can be comprised of stray material that does have electrical charge. When water hits the car, it now has options other than other molecules of water to bond to. While hydrogen bonding will still occur, it will be to a much lesser degree. The water will be spread out and the big water droplets will not be present.
A car is at its best when freshly scrubbed. Oftentimes, this does include a waxing. A lot of other tasks go into ensuring a car runs well.