A Kelp Shake is an addictive and potentially toxic soft drink served in the Kelpshake restaurant chain. They debut as the main plot of the episode "Best Frenemies," and later also make a cameo in "The Nitwitting."
The drink comes in a light green bottle with a light yellow lid and light yellow straw, and on the bottle is a bright yellow label that reads the drink's name in red text.
It is eventually proven by Karen that the drink is 99% kelp juice and the remaining percentile is an unknown radioactive material that, at the end of the episode, is revealed to make the consumer grow green hair on their entire body.
At the end of the episode, SpongeBob, Mr. Krabs, and Plankton are the only ones seen being the victims of the Kelp Shake. However, since almost everybody else in town drinks one, they all suffer the same symptoms.
The body-changing side effects of the beverage cover the entire body in fuzzy kelp similar to a gorilla's fur and their clothes vanish as the fuzzy kelp grows all over their bodies. For some odd reason, however, it takes everybody except for Mr. Krabs and Plankton about two days to grow green hair.
Because of these symptoms, the authorities order the complete shutdown of all of the Kelpshake restaurants and proceed to dispose of the dangerous chemicals used to produce them.
Role in series
The drink was very popular and tasty as Plankton described it as having "a pungent aftertaste." However, Mr. Krabs describes his first sip of it as tasting like a "wet gym sock" before the flavor kicks in. At one point it is so popular that restaurants are on each block of Bikini Bottom. And Than, It had a chemical that can turn anyone’s skin into fuzzy kelp.
Kelp Shakes are listed on the Galley Grub menu inside of the Krusty Krab in "Mermaid Man and Barnacle Boy V" and ironically contradicts the logic of the Krusty Krab banning Kelp Shakes in "Best Frenemies."
However, it is very likely that the Kelp Shake seen on the Krusty Krab menu is a different variant than the one seen in "Best Frenemies."