We’ve all done it – dropped some food on the floor, picked it up, given it a quick wipe and put it in our mouth.
The theory is that if food dropped on the floor is picked up quickly enough, it’s safe to eat.
While some people are firm believers, others use the rule to convince themselves that it's OK to eat a fallen morsel.
Some call it the "five-second rule", others know it as the "three-second rule". Whichever you're familiar with, it certainly isn’t based on science says Dr Ronald Cutler, a microbiologist from Queen Mary, University of London.
To prove what he said Dr Cutler subjected the five-second rule to the rigours of scientific testing.
His trial involved dropping pieces of pizza, apple and buttered toast on different surfaces artficially contaminated with E. coli to emulate what would happen if you dropped food onto a heavily contaminated surface.
The study was to determine if the time the sample was left on the surface affected the degree of contamination picked up.
The idea that dirt is "good" and hygiene somehow "unnatural" has been popularised in the media.
For those who believe that a little dirt never hurt anyone, here’s a sobering statistic – each year in the UK around a million people suffer a food-related illness.
Of these, about 20,000 people require hospital treatment and up to 500 may die as a result.
Many of these cases could have been avoided through basic hygiene such as hand-washing, and preparing and cooking food properly.
Back in the lab, Dr Cutler analysed the food samples and found that they were all covered in germs compared with control samples that had not been dropped.
The samples were dropped on the contaminated surfaces and picked up either immediately, or after five or 10 seconds. Each sample tested was heavily contaminated.
"The five-second rule has little effect on the amount of bacteria you would pick up from a heavily contaminated surface," says Dr Cutler.
"Think about this, if you drop food on a floor, it's better to put it in the bin rather than your mouth.
"No matter if it’s at home on the carpet, the kitchen floor or in the street, my advice is if you drop it, chuck it."
Dr Cutler’s simple test is backed up by other research performed in recent years proving that the five-second rule is a complete myth.
Get 10 tips to prevent food poisoning.