The Whirly Brain is a toy that appears in the episode of a similar name, "Whirly Brains."
The Whirly Brain is a flying brain with eyes, a long stem, and a propeller on the top. The toy's box contains a monster mask to scare someone so you can connect the propeller to the brain to play with it. The Whirly Brain can also produce brain gas out of the stem by pressing the button on the remote control.
Role in episode
In the episode, SpongeBob SquarePants and Patrick Star buy Whirly Brains at the Bikini Bottom Toy Shop after watching an advertisement on television. They use the Whirly Brains to play pranks on Squidward, Mrs. Puff, Mr. Krabs and several other people in Bikini Bottom, until a kid challenges them to a race. But as they play, the Whirly Brains end up getting stolen by a grouchy old man who keeps them in a barrel. With Sandy's help, the Whirly Brains are returned. The old man explains that the reason why he stole them was because he was jealous of seeing kids having all the fun and wanted to experience flight. SpongeBob and Patrick decided to donate the Whirly Brain propellers to the old man and he happily flies around in the air.
Whirly Brains could be a reference to remote control helicopters and toy drones.