If all the world’s women suddenly declared that they would only have sex with men that have six testicles in their one scrotum, and everyone believed it; no one would have any more sex and the human race would disappear within a generation.
If the last step of purchasing a new car were to precisely recite the smell of the color blue, as dictated by an Ancient Eskimo Chief to his son, who died without keeping a record of the description, and everyone believed it; no new cars would be sold.
If the world’s financial authorities declared that the sum total value of all money, gold, silver and other wealth possessed by all of the people on the planet totaled $34 Trillion and that the total amount of debt owed by all of the people on the planet was valued at $84 trillion, and everyone believed it, then money and wealth would be meaningless and worthless, no one would ever actually be able to own anything and everything in life would be measured by what you owe instead of what you are.
Oh shit, wait a minute. That last one’s an actual fact.