So if you drop something on the floor, do you pick it up, brush it off and eat it? Do you follow the rule or not?
URBANA--High-school student Jillian Clarke investigated the scientific validity of the "5-second rule" during her apprenticeship in Hans Blaschek's University of Illinois lab this summer. You know the rule: If food falls to the floor and it's in contact with the floor for fewer than 5 seconds, it's safe to pick it up and eat it.
According to Clarke, a senior at the Chicago High School for Agricultural Sciences, the 5-second rule dates back to the time of Genghis Khan, who first determined how long it was safe for food to remain on a floor when dropped there. Khan had slightly lower standards, however; he specified 12 hours, more or less.
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http://web.aces.uiuc.edu/news/list.cfm?NID=2467