Raised from their stasis-like sleep, the crew of the spaceship Nostromo have stopped to investigate a mysterious distress signal. It's not a good move. In the ruins of an ancient craft in a windswept planet they discover a cargo of giant eggs. John Hurt bends over to peer at one. In a flash, a terrifying organism has attached itself to his face. Taken back to the ship, he remains unconscious until the creature appears to die and fall off. But in the mess hall of the Nostromo he convulses - and a gruesome junior alien bursts from his chest, spraying the onlookers with blood. It's gone down in film legend as the event that Ridley Scott concealed from 28-year-old Sigourney Weaver and her co-stars, hoping to catch real expressions of fear and revulsion.

"As I walked on set," said Weaver later, "I should have noticed everyone [on the crew] was wearing raincoats."

The scene had not been rehearsed. Then Hurt starts screaming. "A small head the size of a man's fist pushes out," reads the script. Contrary to myth, it was not a total surprise to the actors. They had been told to expect something, but no specifics were mentioned. Scott - manning the camera as he did for 80 per cent of the film - was especially delighted by Veronica Cartwright's genuine expression of horror.

The designer of the chest-burster, Roger Dicken, had originally wanted a gorier scene - with the pseudo-foetal alien clawing its way out with its own little hands.

The scene has proved gold dust for parodists - not least for Hurt himself, who reprised it for Mel Brooks in Spaceballs in 1987.