When Spongebob died in The Spongebob Movie. He reproduced and the baby took his place. Meanwhile the original Spongebob got sent to hell which is why Bill is a Interdimensional being. Spongebob was reacarnated as Bill after being possessed by an absract being(*cough* illuminati *cough*) which is Bill exists Trillions Of Years before mankind, so don't try to shoot that one at me. Bill is yellow like Spongebob. Also the Illuminati is green, so why is Bill yellow? Bill has a rather distorted voice. Demons have that, so we need to see how he would sound like without the demonic edge. And if you take it away, it is similar to Spongebob. Bill is immune to the laws of psychics and guess who else is immune? Spongebob! Not to mention Bill can regenerate......Spongebob has the ability of shapeshifting, and so can Bill. Also Bill can possess people via entering their mind, and Plankton one time did that to SB. Just pointing that out. The reason why Bill wants to escape Gravity Falls is to go to Bikini Bottom and kill his old self. Take a look at Bill's clothing. A tophat. The hat looks unnaturally long, similar to Spongebob's hat. And Bill wears a bow, similar to SB's tie. Bill has black legs, and SB has black shoes. And covientally Patrick says Spongebob's shape is a triangle in one episode. Spongebob has a jellyfish net and during the end of Weirdmageddon Part 3, Bill traps Stan and Ford in a net like object. And Spongebob enjoys blowing bubbles, Bill Cipher has bubbles, too, Madness bubbles. So everything Spongebob has is turned dark with demonic powers. Bill Cipher's blue fire is probably the demonic version of Spongebob's stove. Also in one episode, where Spongebob was writing an essay, he fell asleep and the dream seemed to become nonsensable as imminate objects began to talk, which is something that Bill has the ability to do. And during the end of the dream, Spongebob's house began to be set on fire which is Bill's signature abilty. But we don't know what was causing the dream. When his house was sent on fire, the house said "Why did you set me on fire Spongebob?" So it was Spongebob who was causing everything. It was likely a foreshadow of his future self and his fate. Bill seems to be boneless...........The main reason why Bill wants a psychical form is to start a new life, a new job, to become a human. When Bill possess Dipper, he says "I haven't had one of these in a while!" indicating that he had a psychical form before. Also here is a question, what happened to Spongebob after Bill died? Well let me summarize what happened, when Bill was killed by Stan, the illuminati/abstract demonic part of Bill was erased from the psychical and mental world, while Spongebob was turned into stone, here is some evidence at the end of the episode, Bill Cipher's statue is seen in the real world. There's some cartoon character than can exist in the real world and outside the cartoon plane of existance. Spongebob. So Spongebob is now a stone statue stuck in the real world, this time forever. To make it worse, Stan just killed everyone's favorite Sponge.