A tinfoil hat is a pseudoscience method of blocking out alien or government telepathy, mind reading, or remote mind control of a human brain by wearing either a regular hat lined with aluminum foil or to construct a hat out of aluminum foil that covers the skull.
The concept comes from the idea of creating a Faraday cage around a human head to block unwanted electromagnetic or telepathic fields. Many years ago, tin was used for wrapping and packaging until the use of aluminum came into common practice, but the name tinfoil stuck.
The interesting thing is that a student wrote their doctoral thesis on the use of tinfoil hats and discovered that using a beanie made from aluminum foil actually concentrated the electromagnetic signals around the brain.
The first use of this kind of protection came from a short science fiction story by Julian Huxley called "The Tissue-Culture King" in 1926.
Applying the idea that someone wears a tinfoil hat is now a disparaging term that means they are either paranoid or that they believe in wild conspiracy theories.
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