We've all been there -you went to the kitchen, got a snack, and you're headed back to the couch when one of your precious nachos slides off the plate and onto the floor. But, you're quick! And you pick it up after only a second or two. As long as it hasn't been more than five seconds, it's still good to eat, right? Well, not really. The idea that bacteria will wait five whole seconds to climb onto your fallen food -the so-called-five-second rule -is totally made up.

It turns out that what food you dropped and where you dropped it matters more to bacteria than how long it was sitting on the ground. What we know about the five-second rule comes most recently from a pair of food microbiologists at Rutgers University, who were fed up with the lack of solid science on the subject. 

They designed a kind of worst-case scenario for your snack: falling onto a spot that was definitely covered in bacterial colonies. Now, they didn't want to make their experiment dangerous, so they used bacteria called Enterobacter aerogenes, which clings to food the same way Salmonella does, but can't make you sick. The researchers spread these bacteria on four different surfaces: stainless steel, tile, wood, and carpet. Then, they dropped one of four foods -bread, buttered bread, watermelon, or strawberry gummy candies -onto each surface, letting the food sit for different amounts of time, from less than 1 second up to 5 minutes. They repeated each of these scenarios 20 times. That's 2560 experiments -all to find out whether the five-second rule is legit. 

Some bacterial contamination happens after less than one second, so there's no five second buffer protecting your food. And time does matter a bit -the longer food linger on a surface, the more bacteria it will generally pick up. But other factors, like how moist the food is, and the surface it lands on, are more important than time. That wet watermelon, for instance, collected more bacteria than any other snack, and did it almost instantaneously. Meanwhile, the drier gummy candy gathered the least. And even though your old carpet seems nastier than your kitchen's stainless steel, flat surfaces like steel, tile and wood were much better at transferring bacteria. That's because when surfaces are smooth, there's more surface area for the food to make contact and pick up microbes. 

Now, whether you could get sick from the bacteria on fallen food depends on what kind of bacteria are there. But eating food off the floor -or even you kitchen counter or cutting board -would be what the CDC calls surface cross-contamination, which is the sixth leading cause of food poisoning in the U.S. So, if you drop a piece of watermelon in the dirt outside, you might just want to leave that one for the ants. 

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