The Lethargians as seen in the book

The Lethargians are a group of unresponsibly lazy, yet mischevious, dividing slime creatures featured in the book and film "The Phantom Tollbooth", and they live in The Doldrums a dark swampy place filled with what appears to be mucus, to vistors, they pretend to have a law that includes rules for being there such as no thinking, paying attention or laughing allowed, and smiling is permitted only on alternate Thursdays, they spend most of their time lying around, sleeping, or converting from solids to liquids when touching them.

At first glance they appear to be relatively harmless, but they are far more sinister; they sing to their victims to sleep as they sing along with them, and then they trap the victims. Milo the protagonist encounters them while lazily driving down the street in Dictitonopolis ignoring the warning signs, his car gets stuck in a swamp made of mucus and they introduce themselves and pretend to explain the law for being in the Doldrums. When Milo asks one of them what he can do in the Doldrums, if he does not laugh or think, he and the other Lethargians answer his question by singing him their song to sleep, as he joins in, and prepare to surround him and swarm his car by melting it, they are stopped by Tock the Biomechanical Watchdog, in the book they flee in terror and aren't heard from again, while in the movie they return after being chased and turn themselves into a tidal wave in an attempt to kill Milo and Tock, Tock convinces Milo to get his brain working which in turn causes his car to start up again, they evade them by swerving near a cliff and they fall from it presumably to their deaths.