We've all heard of that one right?  You are eating something and you drop it.  If you pick it up within 5 seconds, it's not dirty and is still good to eat.

One YouTube user has put together this little video to highlight just how ridiculous that belief is. 

From the wiki:

...A study on the five-second rule was performed by Jillian Clarke, a high school senior, during a seven-week internship at the University of Illinois at Urbana-Champaign in 2003. Clarke and a doctoral candidate named Meredith Agle took swab samples from various floors around campus. They then looked at the samples under a microscope and discovered that they did not contain significant amounts of bacteria. The conclusion was that in most cases, dry floors would be safe to eat from.

However, Clarke also wanted to test the five-second hypothesis in cases where the floor was known to be contaminated. She therefore spread E. coli on both rough and smooth floor tiles in a laboratory, placed pieces of gummy bears and cookies on the tiles for various amounts of time, and then examined the foods under the microscope. All the foods had a significant amount of bacteria after less than five seconds. Her findings therefore disproved the five-second rule...

...The five-second rule was also featured in an episode of the Discovery Channel series MythBusters. The results they got from their tests confirmed Clarke’s findings: time was not a factor when food is exposed to bacteria; even two seconds' exposure is more than enough time to contaminate it...