Once again, you've dropped your snack. You bend down, snatch it up, and gently blow off any dust—and, you hope, deadly germs. You're about to put it in your mouth because, after all, you've got the "five-second rule" on your side: Food that's been dropped is safe to consume if it's been on the floor for five seconds or less.

But really, should you eat it? Is the piece of toast or the potato chip or the cookie you just rescued from the ground safe to eat, or contaminated by bacteria? Science says ... maybe.

Researchers at Aston University in Birmingham, England, now suggest that the five-second rule is indeed true.

But a 2007 study of the five-second rule from Clemson University in South Carolina argues that there is no safe window for dropped food. Their data points to a "zero-second rule."

Here's the strange thing: Both the Aston study and the Clemson study used nearly identical methods of investigation, and ultimately had the same results—but with staggeringly different conclusions. So is the five-second rule legit or not?

The Science: It's All About Bacteria

When you drop a piece of food on the floor, any bacteria living on the floor will adhere to it. So if you eat the food you've dropped, you're also eating any bacteria the food picked up. Both studies set out to determine how long it takes for bacteria on the floor to stick to food.

The studies tested three different floor surfaces: tile, laminate or wood, and carpet. The Aston study used the bacteria Escherichia coli and Staphylococcus aureus, while the Clemson study used Salmonella typhimurium. Clemson tested only bologna and bread, but the Aston study tested a variety of foods with varying degrees of moisture (a piece of dry toast versus a sticky candy).

The studies agree on several points. When food comes into contact with a contaminated surface, the transfer of bacteria to the food is immediate. And tile, wood, and laminate surfaces transfer much more bacteria than carpeted surfaces. The Aston study found, not surprisingly, that moist foods (cooked pasta or sticky candy) were more likely to pick up bacteria than were drier foods (a cookie or piece of toast).

So Is the Five-Second Rule True?

"It appears that Professor [Anthony] Hilton [who led the Aston study] has substantiated our findings," says Paul Dawson, professor of food, nutrition, and packaging sciences at Clemson University and the scientist behind the 2007 study.

Aston University's Hilton agrees: "Our findings support Professor Dawson's," he says, "in that bacteria are transferred immediately on contact." But, he adds, "the transfer efficacy is extremely low ... hence the five-second rule."

But the two professors disagree as to the degree of contamination.

In Hilton's interpretation, "the initial transfer [of bacteria to food] is insufficient [to contaminate the food]. In our study only one millionth of the bacterial population present on the floor was transferred to the dry food, and approximately 20 times more to the moist. For moist foods, on these flooring types, there is underpinning evidence that fewer bacteria will be transferred to food picked up quickly."

Dawson disagrees. "No matter what the surface or contact time," he says, "enough bacteria was transferred to be detected and to make someone sick."

Would the Scientists Eat It?

Not surprisingly, professors Dawson and Hilton each behave differently when it comes to the five-second rule.

"I compare picking up dropped food and eating it to not wearing a seatbelt," says Dawson. "Someone can drive a lifetime without wearing a seatbelt, never have an accident, and not get injured. Someone can also eat dropped food for a lifetime and never get sick. But in the first case, if you have a serious accident you will probably get hurt. In the second case, if the food you ate was dropped on a surface contaminated with a high concentration of a pathogen, you will probably get sick.

"I still stand by the 'zero-second rule,'" he says. "If I drop food on the floor, I don't see the need to eat it even though the odds are it is perfectly safe."