Almost everyone has dropped food on the floor and still wanted to eat it. Some people apply the "5-second rule" — that random saying about how food won't become contaminated with bacteria if you pick it up off the floor in 5 seconds or less.
The 5-second rule has become such a part of our culture that scientists actually tested it. As you can probably guess, they found that the "rule" is mostly myth: Bacteria can attach to food even if you pick it up super fast. So, depending on which types of bacteria happen to climb on board, you could still get sick.
Here are two facts to consider whenever you feel tempted by the 5-second rule:
A clean-looking floor isn't necessarily clean. A shiny linoleum floor is probably cleaner than a 1970s-era carpet. But even clean, dry floors can harbor bacteria. Newly washed floors are only as clean as the tools used to wash them (picture eating food off the mop in the cafeteria if you need a visual). Even with a brand-new mop or sponge, stubborn germs can still remain on the floor after cleaning.
Fast is better — but it may not be fast enough. Although a piece of food does pick up more bacteria the longer it's on the floor, bacteria can attach to it instantly. So any food that makes contact with the floor can get contaminated if conditions are right. And foods with wet surfaces, like an apple slice, pick up bacteria easily.
When in Doubt, Toss It Out
Some bacteria are not harmful. But others can torture you with miserable stuff like diarrhea. Even if there's no visible dirt on your food, you can still get sick. You just can't tell what kinds of bacteria may be lurking on the floor.
So what are you to do with the piece of watermelon that just slipped from your grip? The safest choice is to throw it out. Or let the dog have it. (And there's another thing to consider — even the 5-second rule can't get around the fact that your food may have landed right in a spot where Fido parked his butt.)