Linguo was a grammar-correcting robot created by Lisa for the science fair. Whenever Linguo detects a grammatical error it responds with the correct answer. Homer nearly destroys Linguo when he mistakes it for a toy robot and gives it beer. Lisa fixes Linguo, but it is later destroyed when members of the Springfield Mafia make numerous grammatical errors in its presence, causing an overload of Linguo's circuits. Linguo's last words are voiced in correction of Homer: "Linguo is dead."
Despite being designed to correct grammar, Linguo was ironically capable of using incorrect grammar itself. When Lisa pointed out that it saying "Sentence fragment" to point out her use of one was itself a sentence fragment, it looked around embarrassed before powering down.
Non Canon Appearance
The Simpsons Game
When Lisa uses her summon in Big Super Happy Fun Fun Game, Linguo appears and explodes, thus destroying the remainder of the Sparklemon's health.
Treehouse of Horror
When Bart is showing Kodos things from Earth, he shows her Linguo.