The five second rule is a myth used to justify eating food that has fallen to the ground. The use of this term is generally accredited to various fast food restaurants which some allege to have endorsed such a policy when preparing food. Those who use the five second rule falsely claim that foods, particularly sweet foods, like cookies, that have fallen to the ground will not contract any germs until five seconds have passed. If they are quickly retrieved from the floor, then the individual will usually mention the five second rule before eating the food in the presence of others. Although it is unlikely that any harm would come from eating food from a relatively clean floor, the notion that germs from the floor would not reach food for at least five seconds is false. The rule therefore seems to have been invented out of convenience, and to prevent others from criticizing the habit of eating fallen food.

The five second rule is seldom invoked in the case of wet and sticky foods, such as ice cream, since dirt often clings to the food, leading to obvious hygiene issues. A somewhat scientific experiment performed by Katriel Ramu and Jean Barker at the University of Maine demonstrated that dry foods were exposed to more bacteria than wet foods when they were on the floor for equal periods of time, contrary to what one would expect. From this, Ramu and Barker concluded that dry foods follow the five second rule, and wet foods do not. They also postulated that if children were permitted to use the five second rule, they would provide challenges to their immune system, which could be beneficial to their health.

The five second rule is sometimes called the ten second rule or the fifteen second rule, to some extent depending on the quality of the food involved.

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The card game Mao also has a 5-second rule in some versions. If a player does not take their turn within five seconds, another player may penalize them.