Is there a foolproof way to announce our existence to other intelligent life-forms? It’s a brainteaser with vast potential consequences, and one that scientists seem to love puzzling over – even if some, such as Stephen Hawking, have questioned the wisdom of alerting advanced beings to our humble corner of the Universe. In this short video excerpt from a 1977 lecture at the Royal Institution in London, Carl Sagan – more optimistic about the prospect of alien benevolence than Hawking – offers one possible method. First, he establishes his belief that a common language among dissimilar beings in a shared universe should, in theory, be possible. He then presents a document coded in ones and zeroes, containing a message that he hopes could be decoded by an intelligent being anywhere in the Universe. Finally, Sagan recounts an experiment in which he presented the document to what he considers a ‘reasonably clever’ life-form – some of his graduate students at Cornell University – to see if they could decipher it.