The five second rule states that when an item of food hits a surface (usually the floor, as a result of being dropped), if it is retrieved within 5 seconds of impact, it is not considered 'dirty', and may still be directly eaten.
The most well known current reference to this rule is embodied in a 30 second Volkswagen Passat television commercial, wherein a new father questions his paternal abilities.
"Suddenly I'm the one saying 'don't touch the cookie on the ground', when I'm really thinking 'five second rule' "
This rule seems to be a rare example of human instinct. It is sometimes referred to as the three second rule, but regardless, it seems to be independently arrived at, in one form or another, by most people.
Of course, the application of this rule varies, depending upon individual preference, the conditions of the food and the surface in a given circumstance, and socio-economic factors.