It feeds on brainwave energy, absorbing all unconscious frequencies and then excreting telepathically a matrix formed from the conscious frequencies and nerve signals picked up from the speech centers of the brain.
13 seconds sound clip from The Hitchhiker's Guide to the Galaxy (1981) - Episode 1 sitcom.
You can hear this line at 00:28:28.400 in the Blu-ray version of the TV Series.
Quote context
[...]
- Excuse me, but what exactly am I doing with this fish in my ear?
- It's translating for you. Look in the book under Babel Fish.
- We're going into hyperspace!
- I'll never be cruel to a gin and tonic again!
- The Babel fish is small, yellow, leech-like, and probably the oddest thing in the universe.
- The speech you hear decodes the brainwave matrix.
- The argument runs something like this.
- I refuse to prove that I exist, says God, for proof denies faith, and without faith I am nothing.
[...]