The formation of a rain droplet begins with the heating of water. As the water gains enough kinetic energy (energy of movement), it evaporates and turns into a gas. As the gas (water vapor) moves away from the surface of the Earth, it cools. This is because temperatures decrease within higher elevations of the troposphere (the layer of Earth's atmosphere that is closest to the Earth). Some of the water vapor condenses around a dust particle and is suspended in the air. This is how clouds are formed. When the condensation around the dust particle becomes a certain size, it becomes too large to be suspended any longer. The water droplet is pulled towards Earth's surface as rain by the force of gravity. If temperatures are cold enough, other forms of precipitation will form (such as sleet, hail, or snow).