The five-second rule is a popular polite fiction regarding the eating of food that has fallen to the floor or ground. The origins of the rule are unknown. The substance of the rule is that if food falls on the ground, it may be safely eaten as long as it is picked up within 5 seconds.
There are many variations on the rule. Sometimes the time limit is modified so that it is known variously as the "three-second rule", "seven-second rule", "the five-minute rule" etc. In some variations, the person picking up the food arbitrarily extends the time limit based on the actual amount of time required to retrieve the food.
Another variation of the rule is that if the item of food dropped is not picked up by the person who dropped it, the item of food can be picked up by anyone, but only after leaving the owner five seconds to pick it up.
Although it has received little scholarly attention, the five-second rule has been studied as both a public health recommendation and as a sociological effect.
In 2003, intern Jillian Clarke of the University of Illinois found in a survey that 56% of the men and 70% of the women surveyed were familiar with the five-second rule, and determined that a variety of foods were significantly contaminated by even brief exposure to a tile inoculated with E.coli; on the other hand, Clarke also found no significant evidence of contamination on public flooring. Clarke received the 2004 Ig Nobel Prize in public health (a parody award) for this work.
A more thorough study in 2007 using salmonella on wood, tiles, and nylon carpet found that the bacteria were still thriving after twenty-eight days of exposure under dry conditions. Tested after eight hours' exposure, the bacteria could still contaminate bread and bologna in under five seconds, but a minute-long contact increased contamination about tenfold (with tile and carpet surfaces only).
The five-second rule was also featured in an episode of the Discovery Channel series MythBusters. They found that time was not a factor when food is exposed to bacteria; even two seconds' exposure is more than enough time to contaminate it. They found out that the maximum time for one to eat food that has fallen on the floor without it being contaminated by germs is only 1.5 seconds.