At the movies, the aliens always look set to destroy all humans and inherit the earth. Then, when the hour is darkest, a wonder weapon - usually some everyday thing we take for granted - is discovered to be toxic to the extraterrestrials, restoring humanity to its rightful place at the top of the food chain.
Warning: Spoilers ahead.
WAR OF THE WORLDS (1953, 2005)
The granddaddy of alien-invasion films and the one that kicked off the trope that aliens always carry a weakness in the last area you would think of. Here, they drop dead in the presence of microbes that are harmless to earth creatures.
INDEPENDENCE DAY (1996)
Scientist David Levinson (Jeff Goldblum) breaks the force field around the alien mothership by uploading a computer virus into the ship's mainframe. This allows American fighter jets to shoot it down and for everyone to wonder if history might have been different if the aliens had used a stronger password.
In this film, written and directed by master of plot twists M. Night Shyamalan, a global invasion is nipped in the bud when the stuff that covers 71 per cent of the earth's surface is found to be extremely toxic to the invaders. Nerds have pointed out that if water is so deadly to aliens, they would have been killed on contact with the humidity in the air.
MARS ATTACKS! (1996)
In this satire of alien-invasion films from director Tim Burton, the idea of the inevitable alien weakness is exploited for laughs.
The bulbous brains of the green-skinned, bug-eyed overlords explode when the Slim Whitman song Indian Love Call is played over loudspeakers.