[i]The Babel fish is small, yellow and leechlike, and probably the
oddest thing in the Universe. It feeds on brainwave energy received not
from its own carrier but from those around it. It absorbs all
unconscious mental frequencies from this brainwave energy to nourish
itself with. It then excretes into the mind of its carrier a telepathic
matrix formed by combining the conscious thought frequencies with nerve
signals picked up from the speech centres of the brain which has
supplied them. The practical upshot of all this is that if you stick a
Babel fish in your ear you can instantly understand anything said to
you in any form of language. The speech patterns you actually hear
decode ...

The Babel fish is small, yellow and leechlike, and probably the
oddest thing in the Universe. It feeds on brainwave energy received not
from its own carrier but from those around it. It absorbs all
unconscious mental frequencies from this brainwave energy to nourish
itself with. It then excretes into the mind of its carrier a telepathic
matrix formed by combining the conscious thought frequencies with nerve
signals picked up from the speech centres of the brain which has
supplied them. The practical upshot of all this is that if you stick a
Babel fish in your ear you can instantly understand anything said to
you in any form of language. The speech patterns you actually hear
decode the brainwave matrix which has been fed into your mind by your
Babel fish.