The Water CycleWater is needed to sustain practically all of life functions on planet Earth. A single drop of this compound is comprised of an oxygen atom which shares its electrons with each of the two hydrogen atoms. The cycle starts when precipitation such as rain, snow, sleet or hail descends from the sky onto the ground. Water not absorbed immediately from the precipitation is known as runoff. The runoff flows across the land and collects in ground water reservoirs, rivers, streams and the ocean. Evaporation takes place when liquid water changes into water vapor, which is a gas. Water vapor returns to the air from surface water and plants. Ultimately, condensation happens when this water vapor cools and changes back into droplets of liquid. In fact, the puffy, cotton clouds that we observe are formed by condensation. When the clouds become so heavily laden with liquid droplets, precipitation then ensues. Which transitional expression best replaces the underlined word in the following sentence:"Ultimately, condensation happens when water vapor cools."