Why is a single-celled organism not classified as an animal?

Single-celled organism are not classified as an animal because animals are multicellular organisms i. e. made of more than one cell while single celled organisms are unicellular organisms i. e. made of one cell. In single celled organisms, one cell perform all activities while in animals special cells are responsible for particular function. There are some single celled organisms are autotrophic while animals are heterotrophic in nature.

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.