You can't touch this

Some modern medical tests use radioactity to trace blood flow during a stress test, thyroid activity, etc. This leaves the patient mildly radioactive for 1-3 days depending on the test or procedure. Under such occasions the doctor will give the patient a card attesting that they have had a medical procedure involving a small amount of radioactive material. Don't lose this card!
You may set off alarms in the airport. Or be stopped at check points to prisons or military bases. Recently a firefighter in Connecticut was pulled over by a policeman after he set off a radioactivity detector in the state police car. The fireman was impressed,  "I had no idea the police even had devices like that." He showed his medical card to the police officer and was allowed to go instead of being held as a suspected terrorist.