In 2007, microbiologists at Clemson University put the five second rule to the test.

They coated squares of tile, wood flooring and carpet with a solution of Salmonella bacteria, waited for it to dry, and then tested how many bacteria transferred from the surfaces to food dropped on them.

After only five seconds on the smooth tile floor, nearly 99% of the bacteria cells had transferred to a slice of bologna. Transfer of bacteria from wood floor and carpeting was lower; after five seconds five to seventy percent of the bacteria had transferred to the bologna.

Although a slice of bread picked up fewer bacteria than the bologna did, it still was contaminated with loads of bacteria after five seconds on the floor.