Stockpile ordinary stuff like water
Despite clear technological superiority, it is possible that aliens might be thwarted by the simplest substances on our planet. In the film “Signs,” aliens initiate an elaborate invasion that doesn’t take into consideration that our water is toxic. Humanity prevails.
But not all aliens would be so easily dissuaded. Aliens have taken many biological forms throughout history, but the predominant class of aliens sighted today are called “grays.” Based on alleged abductees’ testimonies, expert Frederick V. Malmstrom reports that grays generally appear short and gray. Further, “80% of our sample [of reported aliens] had the typical UFO face: prominent, somewhat diagonally oriented eyes, double-slit nostrils, and little or no evidence of a mouth.”