A tin-foil hat is a piece of headgear made from one or more sheets of aluminium foil or similar material. Some people wear the hats in the belief that they act to shield the brain from such influences as electromagnetic fields, or against alien interference mind control and/or mind reading. The concept of wearing a tinfoil hat for protection from such threats has become a popular stereotype and term of derision; the phrase serves as a byword for paranoia and is associated with conspiracy theorists.