How can flies transfer germs between surfaces?

1. A fly buzzes into the kitchen and heads for the sandwich. People always shoo flies away from food. What is the problem with a few tiny footsteps?
2. Here is a fly's foot, magnified. It has two claws and two soft pads covered in tiny hairs. The pads are a little sticky, so a fly can walk up glass and across the ceiling.
3. We sprinkled harmless pink marker dust on the compost. The fly lands there to look for food, and the dust sticks to its feet and leg hairs.
4. Now watch where the pink goes. The fly lands on the counter, then the sandwich, then the apple. It leaves some markers each time, a little fewer with every landing.
5. Real germs are far too small to see. Most germs are harmless, but a few kinds can upset your tummy. Since you cannot see where a fly has walked, a few good habits do the job.
6. So keep the lid on the bin and a cover over food. Wipe the counter. And wash your hands with soap before you eat. Soap and water send the germs down the drain.

Tap each part of the fly. Find the feet to see why things stick to them.

• Eyes. Each big eye is made of thousands of tiny lenses. A fly sees almost all around itself and notices movement very fast. That is why it is so hard to catch.
• Antennae. Two short antennae smell the air. They help the fly find food from far away.
• Sponge mouth. A housefly cannot bite or chew. Its mouth ends in a soft sponge that dabs up liquids. It has to wet dry food first to turn it into a drink.
• Wings. A housefly has one pair of wings that beat about 200 times every second. That makes the buzzing sound.
• Bristles. A fly is covered in stiff little hairs called bristles. They feel air movements, and tiny bits of dust and dirt get caught in them.
• Legs. Six jointed legs let a fly walk on any surface. You may see a fly rub its legs together. It is cleaning them.
• Feet. Each foot has two claws and two sticky pads, so a fly can walk upside down. Flies also taste with their feet. Tiny things on a surface stick to the pads.

Send the fly to the pink marker dust, then tap other things to make it land on them. Count the markers it leaves each time.

Experiment: The fly lands on the marker dust, then on the sandwich, the apple and the cookie. Which food gets the most markers?

1. Tap the compost bin so the fly picks up pink markers.
2. Tap the sandwich, then the apple, then the cookie.
3. Compare the numbers above each food.

What we found: The first thing the fly lands on gets the most markers, because its feet are fullest then. Each landing after that gets fewer. But every landing gets some, so one dirty stop can reach many places.

Turn on JavaScript to try it yourself.

The fly will visit the bin, the counter and all three snacks. You may use two covers or lids. Keep every snack free of markers, then wash your hands. For 3 stars, use only one.

A housefly's feet are amazing. Each foot has two claws for gripping rough things and two soft pads covered in tiny hairs. The pads make a thin, sticky liquid, which is why a fly can walk up a window or stand upside down on the ceiling.

Sticky feet pick things up. Flies look for food in places like compost, bins and soil, where there are lots of germs. Germs are living things too small to see. When a fly lands there, some stick to its feet and to the stiff hairs on its legs.

When the fly lands again, some of what it is carrying rubs off. In our kitchen we used a harmless pink marker dust so you could see it happen. The first place the fly lands gets the most, and each place after that gets a little less.

Most germs are harmless, and many are helpful. A few kinds can upset your tummy if they get into food. So people use simple habits: keep a lid on the bin, cover food, wipe the counter, and wash hands with soap before eating. Soap loosens germs from your skin, and the water carries them down the drain.

Key words

Germ

A living thing too small to see. Most are harmless, but a few can make you unwell.

Carrier

An animal that moves tiny things from one place to another on its body.

Marker

A harmless colored powder used to show where something has been.

Transfer

Moving from one surface to another by touch.

Source

The place where something starts out.

Check your thinking

1. A fly lands on marker dust and then on a sandwich. What happens?

   Show the answer

   Some markers rub off its feet onto the sandwich. Tiny things stick to the fly's feet, and some rub off at the next landing.

2. Why do we wash our hands with soap before eating?

   Show the answer

   Soap and water lift germs off our skin and rinse them away. We cannot see germs, so we wash them off before they can get onto our food.

3. A fly visits the marker dust, then the plate, the cup and the spoon. Which gets the fewest markers?

   Show the answer

   The spoon, because the fly's feet have fewer markers left by then. Each landing uses up some of the markers, so later landings leave fewer.

4. There are three snacks on the counter and a fly nearby. Which one action protects all three?

   Show the answer

   Putting the lid on the bin, so the fly cannot pick anything up. If the fly never touches the dirty place, it has nothing to leave behind anywhere.

5. The glitter comparison works for markers. In what important way are real microbes different from glitter?

   Show the answer

   They are alive and can multiply on food that is warm and moist. Glitter only spreads. A few living microbes can become many, which is why food is covered and kept cool.