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Aluminum foil hats.  Some people think aluminum foil hats will stop electromagnetic radiation and other signals coming from alien sources. Have two students come to the front and call one another using cell phones.  Make sure the ringer is on.  The phone rings and the student answers.  Now wrap the cell phone in aluminum foil.  Be sure the foil is relatively new and has no pinholes in it.  Have one student call the other again.  No luck. The aluminum does stop the cell phone microwaves.  If you are afraid the new drivers licenses required by the federal government will allow you to be tracked, wrap it in aluminum foil.