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Castform is an artificial Pokémon that changes appearance and attitude based on the weather. Its body is made of cells exactly like those of water molecules, causing its cellular structure to be chemically altered by temperature and humidity. Though not of its free will, Castform uses this ability to protect its tiny body. In its Normal form, it resembles a plain, gray cloud with a single wisp extending from the top of its head.

Regardless of the form it takes, it will always have a large, spherical head and a lighter-colored mask-like marking over its eyes. Various experiments were conducted to change its form in artificial conditions, but all have failed. Despite its man-made origins, it can be found in grassland areas.

Forms
Castform has three alternate forms: Sunny Form, Rainy Form, and Snowy Form.

Under harsh sunlight, it takes on its Sunny Form. Its lower half becomes pure white, while the head turns orange has develops small orange spheres around it. This gives it the overall appearance of the sun above a small cloud. Its skin glows and dries, and becomes hot to the touch.

When it is soaked in rain, it takes on its Rainy Form. It has a dark gray lower half resembling a storm cloud. The head turns blue and resembles a raindrop. Its body is soft and slippery and seeps with water when compressed.

When it is hit by hail, it takes on its Snowy Form. Its head turns purplish-blue. The rest of the body changes into the shape of a mint green, cumulonimbus cloud. Its body becomes a smooth ice-like material, partially frozen measuring temperatures near 23 degrees Fahrenheit.

(Source: Bulbapedia)