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In its natural state, Ditto is a light-purple or pink blob-like Pokémon with vestigial facial features. It is often referred to as amorphous, but has a relatively consistent appearance in official artwork, including two small nubs on its "head", a few soft lumps at its base, and two pseudopod-like protrusions in place of arms. The face consists of beady eyes and a simple mouth; almost always pulled into a smile.

Ditto bears a variety of unusual and often entirely unique traits that set it apart from every other Pokémon. It is capable of transforming into an exact replica of any physical object or living creature, including its form and abilities. Each Ditto has its own strengths and weaknesses when it comes to transforming; being unable to remain transformed while laughing and getting details of its transformation wrong if based on memory being apparently universal. The Pocket Monsters Carddass Trading Cards depict Ditto being able to transform separate sections of its body into parts from multiple Pokémon at once. The anime, TCG, and some spin-off games have shown that occasionally Ditto cannot change its face during transformation; a trait that has become synonymous with the species despite being incongruous with their depiction in core series games. While its transformation ability allows it to get along with almost anything, it does not get along with its own kind, which it will quickly (and generally fruitlessly) attempt to transform into. It is shown to be able to transform into humans as well; however, it cannot mimic human speech. It transforms into a rock when sleeping to avoid attack.

(Source: Bulbapedia)