A long standing rule in which any food which is dropped on the floor/ground is still "good" and edible if it was only in contact with the surface for less than 5 seconds. Incorporated mainly by clumsy children but is often used by teenagers and adults as well.
The tacit requirement in ultra-clean American society that dropped food must not lay upon the ground for longer than five seconds in order to be still edible. Certain requirements about food type (i.e., non-sticky or attracting of dirt particles) generally apply.
Dude! I dropped my Dorito! Five second rule...still good.
The '5 second rule' applies when food comes into contact with an object that is tainted. If it is removed from the tainted object in 5 seconds or less, then the food is considered safe for human consumption and eaten as if nothing happened. If it stays on the tainted object for more than 5 seconds then it is considered unfit for human consumption and is discarded.
I'll bet that fat kid eats the cookie he dropped on the floor even though the 5 second rule is up.