Jake the Dog is one of the main characters of the cartoon series Adventure Time. He is the best friend/companion/adoptive brother of the series' protagonist Finn. A "magical dog", Jake possesses a human-like intelligence and lifespan. He also possesses an ability unique to him: he is able to stretch and contort his body in virtually any way he chooses and is capable of shape-shifting and altering his size. This is due to him being conceived from a magical pustule inflicted upon his father by a shape-shifter's bite.
Jake was conceived when his father, Joshua, was bit by an extra-dimensional creature during an investigation with his wife Margaret. Jake was later born from the pustule on his father's head from the creature's bite and was taken by Joshua and Margaret to live alongside their natural-born son Jermaine. It is inferred that Jake's Stretchy Powers that he has had since he was a puppy are a result of his biological relationship with the shape-shifting creature although according to him, he rolled around in a magical mud puddle when he was a puppy then just became a magical dog. Little is known of Jake's past with his family, but for a while he was known to be a successful criminal until he retired to take part in a more respectable life as an adventurer with Finn, later settling down with Lady Rainicorn to become a father.
Jake's Stretchy Powers enable him to modify the size, shape, and dimensions of every part of his body. Not only can he stretch and contort himself into highly specific forms, but he can also rearrange his internal organs, and teeth. One frequent use of his powers is Key Hand, which he uses to pick locks. He has also used his arm as a bow able to accurately shoot arrows at a short distance.
Although he lacks the zeal for fighting that Finn possesses, Jake is a capable fighter. What he lacks in Finn's talent for swordsmanship, he makes up for in toughness and magical brawn. He is a capable hand to hand fighter, and can transform his limbs into weapons. He can also stretch to entangle enemies, or grow to gain a height and weight advantage. His shapeshifting can grant him supernatural strength, allowing him to simply overpower his enemies. However, Jake uses his powers lazily and does not know their full extent. Despite stating outright that he's multiple times stronger than Finn, Jake often leaves the brawling to his adoptive brother.
Jake is also able to stretch himself to "create" an entirely new person, as long as they remain connected to him. He is shown to be able to move objects through his body freely when he lets Princess Bubblegum jump into his mouth and then pushes her up through his back. Jake can be stretched by force and his powers are sometimes used against his will. His stomach seems to have a will of its own as it once took the shape of a fist and punched him so that he would wake up and eat.
Jake's stretching ability is subject to the principle of conservation of mass. However, the ratio at which he retains mass is different than what applies to other masses in Ooo. It is revealed that while Jake can stretch his body to incredible lengths, his body could be pulled so long that it becomes dangerously thin. Once his mass is distributed along a certain length of his body, it becomes difficult for him to support himself; at one point, Jake was forced to walk along the ground. If he were to stretch beyond his absolute limit, it is presumed that he would completely thin out and die. Without his magical powers, Jake is rather weak and can barely even run.
Being a magical dog, Jake has an acute sense of smell, as he can apparently smell the precise location of an object miles away. He has stated that his sense of smell is "1,000 times better" than Finn's, although compared to normal dogs this is a low estimate, suggesting that he was either just guessing, or his sense of smell is similar to that of an average dog.