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Spoiler warning: Plot or ending details follow.

"Remember how we always thought there wasn't a way to kill a toon? Well Doom found the way: turpentine, acetone, benzene; he calls it the Dip!"

―Lt. Santino.

The Dip (aka Toon Acid) is a green, ghastly chemical invention seen in Who Framed Roger Rabbit. It was created by Judge Doom and is feared by all toons as it's Doom's preferred method of passing punishment.

According to Lieutenant Santino, it's a mixture of turpentine, acetone, all of which are paint-thinners, and benzene, a natural constituent of crude oil, and one of the most basic petrochemicals. While relatively harmless to humans, any toon that comes in contact with it will melt slow and painfully, and is apparently the only surefire way to kill a toon (although Eddie Valiant gets most of The Toon Patrol members to laugh themselves to death).

Ironically, Judge Doom ends up being dipped himself near the end of the film, after revealing himself to be actually a toon. He invented a machine, called "The Dipmobile".

Trivia

The Nostalgia Chick placed it at Number 1 of the Top Ten Disney Deaths.
The Dip is the Toon equivalent of the human death penalty.
According to another crossover film, Wreck-It Ralph, if a video game character dies in a game that he or she is not native to, the character will not regenerate like normal. This could be considered the video game equivalent of the Dip.

Who Framed Roger Rabbit