Scientists have actually investigated the Five-Second Rule.
The Five-Second Rule states that it's okay to eat a dropped piece of food, as long as you pick it up before you can count to five.
In 2003, a high school intern at the University of Illinois named Jillian Clarke conducted a survey and found that half the men and 70% of the women knew about the five-second rule. Jillian then conducted this experiment: first, she contaminated a ceramic tile with E. coli bacteria, then she
placed gummy bears and cookies for 5 seconds and analyzed the food: they sure were contaminated.
In 2007, professor Paul L. Dawson of Clemson University and colleagues repeated the experiment. This time, the test surfaces were tile, wood flooring, and nylon carpet; the food were bread and bologna; and the bugs were salmonella. They discovered that a) salmonella can survive for 28 days on a surface and b) in just 5 seconds, anywhere from 150 to 8,000 bacteria transferred to the food.
So what does this mean for the Five-Second Rule? Do what you like, but remember: the infectious dose (the smallest number of bacteria that can actually cause illness) is as low as 10 for salmonella and 100 for E. coli. (Source)