Most people have heard of the five-second rule. It says if you drop a piece of food, such as a potato chip or an apple on the floor, and if you pick it up right away, within five seconds, it is still clean enough to eat.
Our science class at George Washington Community School decided to test this rule. No, we didn’t use each other to test the rule by eating dropped food to see who would get sick. We used the scientific method to perform an experiment to find our answer.
Experiments should always have a set of variables, the dependent variable and the independent variable. It should also have constants and a control. We used equal amounts of nutrient agar in Petri dishes as a constant. Incubation temperature and time of incubation was a constant. The control was a clean-agar dish that had not been smeared with dropped food. Our independent variable was the time we left the food on the floor – one piece for more than five seconds, the other piece for less than five seconds. The dependent variable was the number of bacterial colonies that grew on the agar after 24 hours.
We decided that we would use bite-size Hershey chocolates as one of our constants. We would simulate the dropping of the candies by rubbing one on the floor for three seconds and another on the floor for seven seconds. If the five-second rule is correct, the hypothesis would be that the candy left on the floor for only three seconds should contain significantly fewer bacteria than the candy left on the floor for seven seconds. However, some of the students decided to state a different hypothesis, that both candies would contain approximately equal numbers of bacteria.
Our procedure, after rubbing the candies on the floor for the specified amount of time, was to rub them on top of nutrient agar in the Petri dish for five seconds each. We then incubated the Petri dishes for 24 hours at about 85 degrees F. At the end of the incubation period, we found that both the five-second dish and the seven-second dish contained considerable numbers of bacterial colonies. We performed three trials, one for each class period. In two trials, the number of colonies was approximately equal. In one trial, the three-second dish actually contained more colonies than the seven-second dish. The control dish contained no bacteria.
We, therefore, concluded that the five-second rule was bogus and simply a myth. The hypothesis stated by students who said that both pieces of candy would contain considerable bacteria proved to be correct. It is not wise to eat any food dropped on the floor for any amount of time.
See a video of one student, Emily, performing the experiment on YouTube.