As many mothers have long suspected, teenagers really are strangers to soap.
Ultraviolet light shows up bacteria on a boy's hands
A survey has found almost half do not always wash their hands after they have been to the toilet at school or before eating, putting them at risk of food poisoning.
The most common reasons for not washing their hands were not having time, because the toilets were dirty or simply because they forgot.
The survey of over 500 teenagers aged 11 to 19 by the Food and Drink Federation (FDF) found 40% of those who do always wash their hands don't always use soap.
We clearly need to keep plugging away with simple food hygiene messages
Martin Paterson, Food and Drink Federation
Three quarters say they dry their hands on their clothes if in a hurry.
But the FDF warns 1,000 times as many germs are spread from damp hands than dry.
Over half the teenagers surveyed said they did not always wash their hands before eating lunch at school, even though two-thirds had a meal such as a packed lunch or burger and chips which they had to eat using their hands.
But many teenagers did know the rules about when they should wash their hands.
Two thirds knew they should wash them after going to the toilet, and 41% that they should have clean hands before preparing food.
But only a quarter knew they should wash their hands after touching raw meat and before touching other foods or utensils.
Just one in third knew it was important to wash their hands after touching pets because they could harbour germs.
Martin Paterson, deputy director general of the Food and Drink Federation, said: "Kids' handwashing habits are a potential recipe for disaster.
"Whilst most teenagers understand how food poisoning can be caused, they're not taking the simple precautions to prevent it."
He added: "Of course teenagers don't want to be nannied, but with up to 5.5million cases of food poisoning a year, we clearly need to keep plugging away with simple food hygiene messages in fun and imaginative ways, to remind people of all ages how they can continue to enjoy their food safely."
Hugh Pennington, professor of microbiology at Aberdeen University said: "The best news about food safety is that one doesn't have to be a rocket scientist to do it properly.
"The bad news is that even a simple and obvious thing like hand washing - which protects incredibly well - is seen by many people as good health theory, but not actually put it into practice."
Judith Hilton, head of food safety for the Food Standards Agency, said: "Handwashing is a simple thing everyone can do to reduce the risk of poisoning themselves or others.
"The survey of young people echoes the findings of the 2002 FSA survey or catering workers, which showed 39% did not wash their hands after going to the toilet and 53% did not do so before they prepared foods."