You’ve probably heard your students say “five-second rule” when they drop food on the floor. They quickly pick it up and eat it.

Theoretically, if a piece of food is on the floor for less than five seconds, no bacteria will get on it and it will still be safe to eat. But science has proven what we’ve long suspected: the five-second rule is wrong. Bacteria can move to food very quickly.

Professor Donald Schaffner at Rutgers University worked with one of his graduate students, Robyn Miranda, to put the five-second rule to the test. They introduced bacteria to five surfaces: stainless steel, ceramic tile, wood, and carpet. Then they dropped watermelon, bread, buttered bread, and gummy candies onto those surfaces. They picked those items up after one, five, 30, or 300 seconds to see how much bacteria got onto the food.

There is some variation, with carpet being weirdly resistant to transferring bacteria, while watermelon picked up bacteria fastest. All told, they used 128 different scenarios, 20 times each, for a total of 2,560 measurements. That’s a lot of dropped food, but also a lot of data. And that data tells us that the amount of bacteria that gets on your food when you drop it varies depending on what kind of food it is, the surface it’s been dropped on, and how long you leave it. One thing is for sure: there is no safe amount of time to leave food on the floor.

The bacteria they used in the tests were harmless, but they got onto the food regardless, even when it was picked up within one second. The difference is that the longer the food was in contact with the “floor,” the more bacteria were on it.

“The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food,” says Schaffner. “Bacteria can contaminate instantaneously.”

What it comes down to is, if you thought the five-second rule sounded silly and impossible, you were right. If you drop food on the floor, by all means pick it up—but put it in the compost, not your mouth.

Testing the five-second rule might make a really interesting science lab project. Would you perform a similar experiment in your students’ biology lab? Have you done so? Please share your ideas for related experiments in the comments.