A bit of a tangent, but educational none the less....
From the wonderful world of Wikipedia:
Tin-foil hats and mental illness
There have been some people who believe in the efficacy of tin-foil hats and similar devices. Reasons for use include preventing abduction by alien beings, or stopping unpleasant experiences such as hearing voices in one's head. This draws on the stereotypical image of mind control operating by means of ESP, microwave radiation or other technological means. In some cases, belief in tin-foil hats could be a manifestation of a disorder such as paranoid schizophrenia.
The delusion of "mind control rays" or other invasive mental activity may seem very real to those afflicted with severe paranoid delusions, and such persons have been known to make and wear improvised defences against the imagined invasion. A placebo effect may even convince the sufferer that the device actually works. While aluminium foil or tin-foil is traditional, less fragile materials such as 3M Velostat (a kind of metallised plastic) and metal window-screen mesh are now more commonly used. Electrical conductivity is seen as a key quality.
There is a small amount of truth or reason to be found in the tin-foil hat story. A well constructed tin-foil enclosure would approximate a Faraday cage, reducing the amount of (notionally harmless) radiofrequency electromagnetic radiation inside. A common high school physics demonstration involves placing an AM radio on tinfoil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the tin-foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimeter-thick tin-foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked. The effectiveness of the tin-foil hat in stopping radio waves is greatly reduced by the fact that it is not a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be reflected and partially 'focused' towards the user's brain. While tin-foil hats may have originated in some understanding of the Faraday cage effect, the use of such a hat to attenuate radio waves belong properly to the realm of pseudoscience.
