Now scientists have tested the trusty “three second rule” and found that processed foods are less likely to harbour harmful bacteria.

Scientists from Manchester Metropolitan University dropped bread with jam, cooked pasta, ham, a plain biscuit and dried fruit and left them all on the floor for three, five and ten second intervals.

The processed foods – ham, bread and jam – did well in the tests and when they were retrieved from the floor after three seconds, they showed little bacteria growth.

Biscuits were also found to be relatively safe after they were dropped for three, five and ten seconds because of their low water content.

The study showed that the biscuits and processed food took between five and ten seconds before harmful bacteria started to form.

But the dried fruit and coked pasta showed signs of klebsiella bacteria after three seconds, which can cause pneumonia, urinary tract infections, septicaemia and soft tissue conditions.

Technical officer Kathy Lees said: “No specific organisms were detected on the biscuit, which has a low water activity level and low adhesion ability.

“Ham is a processed meat preserved with salt and nitrates which prevents the growth of most bacteria.

“The cooked pasta had a slightly increased yeast count after five seconds and very low levels of Klebsiella were detected at all contact times, three, five and ten seconds.”

A baby’s dummy didn’t do too well in the test, picking up traces of E coli within three seconds.