BABEL FISH

 

"The Babel fish is small, yellow and leech-like, and probably the oddest thing in the Universe."

―The Hitchhiker's Guide to the Galaxy

This small non-sapient being lives in the brain of other creatures and feeds on "brainwave energy received not from its own carrier, but from those around it". By unintentionally converting sound waves into brain waves and excreting those into the mind of its host, it actually works as a universal translator and enables its carrier to understand any spoken language, no matter how strange.

Source: The Hitchhiker's Guide to the Glalaxy