The five-second rule, sometimes also the three-second rule, is a western cultural food hygiene concept, that states that there is a defined window where it is permissible to pick up food (or sometimes cutlery) after it has been dropped and thus exposed to contamination.
Some may believe this assertion, whereas most people employ the rule as an amusing social fiction that allows them to eat a dropped piece of food, despite the potential reservations of their peers. How many and what type of bacteria would stick to a piece of dropped food depends on many factors, the food or the floor being wet or dry among them.
There is also a social dimension as dropped food in a restaurant or when guests are around is simply unacceptable, but in a family or private situation it may be still tolerated.
A survey of 2,000 Americans found 79% admitted to eating food that had fallen on the floor.
There appears to be no scientific consensus on the general applicability of the rule, and its origin story is unclear.