When food falls on the floor, some people pick it up quickly and say, "Five-second rule!" They think the food is still safe.

Scientists at a university in the US tested this idea. They dropped food on the floor 2,560 times. Bacteria moved onto the food in less than one second. Wet food, like watermelon, picked up the most bacteria. So the rule is not true. Bacteria do not wait for you.

Food scientists at Rutgers University dropped food on the floor again and again, on purpose. They wanted to test the "five-second rule," the idea that food is safe if you pick it up quickly. They used four kinds of food, including watermelon and gummy candy, and four surfaces, such as tile, wood and carpet.

The food stayed on each surface for different times, from less than one second to five minutes. In total, they took 2,560 measurements. Bacteria moved onto the food in less than one second.

Longer contact meant more bacteria, but no amount of time was safe. Moisture mattered most. Watermelon picked up the most bacteria, and gummy candy picked up the least. Carpet, oddly enough, passed on fewer bacteria than tile or steel.

The "five-second rule" suggests that food retrieved quickly from the floor has not had time to become contaminated. A team led by food scientist Donald Schaffner at Rutgers University put this idea to a systematic test. They combined four foods, namely watermelon, plain bread, buttered bread and gummy candy, with four surfaces, namely stainless steel, tile, wood and carpet, and four contact times ranging from under one second to five minutes, plus two ways of preparing the bacteria.

Each of the 128 combinations was repeated 20 times, producing 2,560 measurements. Bacteria transferred in less than a second, and although longer contact generally increased transfer, no threshold separated safe from unsafe. Moisture proved more influential than time, since bacteria travel mainly through water; watermelon was the most contaminated, gummy candy the least. Carpet transferred surprisingly little, probably because its fibers reduce contact.

Another study showed that Salmonella can survive on dry floors for weeks. That said, the experiment used a harmless substitute bacterium applied at high concentrations, which may not reflect real floors. The real risk therefore depends on how dirty the floor is and how vulnerable the eater is.