When inserted into the ear, its nutrition processes convert sound waves into brain waves, neatly crossing the language divide between any species you should happen to meet whilst travelling in space.

Some say that the evolution of the Babel fish could not have been accidental, and hence that it proves the non-existence of God.

Arthur Dent, a surviving Earthling, commented only 'Eurgh!' when first inserting the fish into his ear canal. It did, however, enable him to understand VogonPoetry - not, necessarily, a good thing.