The best indication that dreaming serves a necessary biological function is provided by the fact

most organisms, especially mammals experience what is known as REM rebound.

Explanation:

An increasing frequency of rapid eye movement [ REM sleep] that occurs after period of sleep deprivation is regarded as REM rebound.

The REM sleep is described to be important for normal brain development especially in the infants, because ,it stimulates part or region of the brain used in learning. This is why, infants are believed to spend more time in REM sleep than the adults.

Note that, REM means Rapid Eye Movement.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.