SpongeBob's brain is an internal organ that first appears in the episode "Plankton!"
It has many different appearances. It is most commonly pink. Depending on where it appears (including episodes, movies, and shorts), his brain can look and function quite differently.
Role in series
Plankton invents a device that allows him to control others' brains. It works at least partially on SpongeBob's brain, although SpongeBob seems to still be in control of his thoughts and some of his actions.
It is mentioned and seen in Squidward's thoughts. When SpongeBob and Patrick claim they will knock each other's brains out in a snowball fight, Squidward imagines the scenario, in which Patrick throws a snowball at SpongeBob and his brain falls out.
The inside of SpongeBob's brain is seen in this episode. Inside, there are miniature figures resembling SpongeBob, and there are areas resembling an office and a bathroom. When Squidward pretends to own a five-star restaurant, the miniature SpongeBobs delete all of SpongeBob's thoughts and memories so he will only think about being a successful waiter.
Plankton has a device that is able to play music from different formats; one of them is "brain," and, in hopes of hearing a song about the Krabby Patty secret formula, Plankton connects SpongeBob's brain to the machine. This results in SpongeBob singing the music selected from the machine dial.
The brain falls out of SpongeBob's head by Mr. Krabs' search of SpongeBob's 'penny'. They both chas it, showing that SpongeBob can still move and function without it. The brain also has the ability to squirt some sort of liquid at SpongeBob.
When Squidward trying to make SpongeBob cry, the inside of SpongeBob's brain is shown. The brain apparently has a large dam built inside it. SpongeBob's tears cause it to collapse and the tears overflow the city of Bikini Bottom, being a metaphor for SpongeBob being no longer able to hold in the tears.
Plankton once again goes inside it, and it once again is dark pink and smaller. Inside the brain is a gigantic space with SpongeBob's extremely happy thoughts, including the popsicle twins. This huge amount of happiness drives Plankton insane.
This episode revolves around SpongeBob's and Patrick's brains. They both use their own brains as Whirly Brains, an invention where one's own brain can fly and be controlled in the same way that a normal person would to a remote-control helicopter.