Key concepts
Heat transfer
Thermal insulator

How lovely it is to come home after a chilly winter walk to a cozy house, put on your fluffy slippers and settle by the fireplace with a warming cup of hot chocolate? Animals such as the polar bear, Arctic wolf or Antarctic penguin are not so lucky to have such a place to keep warm. How do they face the extreme winter temperatures? Most grow “winter coats” in the fall. Did you ever wonder what these coats look like, and how they help keep animals warm? Do this cool activity and you will feel it firsthand!

Warm-blooded mammals—including humans—use part of the energy they get from the food they eat to keep their bodily temperatures relatively constant. A lot of energy is needed to regulate your temperature, especially when your environment is way cooler than your body.

Humans use clothes to keep themselves warm—and they need even more layers of clothing the colder it is. Clothes form a thermal barrier between our warm bodies and the cold weather, preventing heat from escaping. Other mammals have developed natural barriers to trap body heat, allowing them to survive in cold climates. It is as if they take on several “winter coats,” each of which plays an important part.

The first coat is often a layer of fat hidden under the skin called “blubber.” In addition to being a good thermal insulator—it does not allow heat to pass through it easily—fat has the added benefit of serving as a welcome food reserve. The next coat, depending on the animal, is often a dense layer of underfur or down feathers packed closely against the skin like a warm undershirt. This cozy, soft coat is filled with air pockets that provide insulation; a stationary layer of air is another good thermal insulator. Finally, to keep out moisture, there is often a layer of oily, water-repellent guard hairs or feathers. Sometimes these hairs appear white but they are often transparent and hollow, which has a big advantage: they provide extra thermal insulation.

Some arctic animals, such as polar bears, are so well insulated that they cannot be spotted with night-vision goggles, which pick up radiated heat. This implies their outer layer of fur has the same temperature as their surroundings. In other words, these animals can keep all of their heat inside, losing none to the environment. How amazing!

Ready to feel the insulating capacities of these materials? Gather a few items and try it out!


  • Zip-top sandwich bags (four)
  • Small plates (four)
  • Ice cubes
  • Small feathers (Craft feathers are fine.)
  • Quarter pound of butter (one stick) at room temperature
  • Ruler


  • Fill one sandwich bag with a layer of feathers about two centimeters thick and close the bag.
  • Fill another sandwich bag with a two-centimeter-thick layer of butter and close the bag. If the butter was not at room temperature, leave the bag and butter out for awhile before you perform the test.
  • Trap air into a third sandwich bag. Try to make it as thick as the other bags. (Closing the bag a little over halfway first, then blowing into it and closing the remainder works well; because the air you trap this way is warmer than room temperature, it is best to leave the bag out for awhile to adjust.)
  • Leave a fourth sandwich bag empty.
  • Place a layer of ice cubes on four small plates.
  • Place one bag on top of the ice cubes on each plate.


  • Let one hand rest on the empty bag, the other on the feathered bag. Avoid pressing down on the bags. Which one feels colder?
  • Remove your hands and wait a moment until both hands feel warm again. Now, place one hand on the empty bag and the other on the bag filled with fat. Which one feels colder now, the empty bag or the bag with a layer of fat?
  • Before you go on, which one do you expect to feel colder, the empty bag or the bag with air—or would they both feel as cold?
  • Feel which one feels colder. Was your prediction correct?
  • In a moment you will compare the layers of feathers, fat and air. Which one do you think will feel warmest and which one will feel coldest? The one that feels warmest creates the best thermal barrier and thus, is the best thermal insulator. Scientists call materials that prevent heat transfer “thermal insulators.”
  • Perform the test. Make sure to let your hands adjust to room temperature before comparing. Were you able to tell which bag felt warmer or is the difference too small to detect?
  • For a more detailed test, leave the bags on the ice cubes for 15 minutes and feel again. Are you able to detect a difference now?
  • Do your predictions and your test results agree?
  • Extra: Do you think adding several layers will create a bigger thermal barrier? To test this, you first must let your bags warm up again. Take them off the ice and leave them at room temperature for awhile. Then place a stack of two bags on ice cubes and measure how long it takes before the top feels cold. Does it take longer than with the single layers?
  • Extra: Try some other materials, such as wool, cotton, leather, fleece, paper or aluminum foil. Which ones do you expect to be good insulators? Do your test results agree with your predictions?
  • Extra: Now that you know some good thermal insulators, could you design a survival suit for a scientist working in the Arctic or a mountain climber exploring Mount Everest? Once you are done with your design, look up what people living or working in extreme cold conditions wear. Is your design similar or very different? Why do you think they chose these outfits?

Observations and results
The empty bag probably felt a lot cooler than the filled ones. The materials in the bags are good thermal insulators—they do not let heat travel through them easily. Stacking two bags on top of each other creates an even stronger barrier, so it probably took longer before the top bag felt cold.

The bag with butter (a fat) probably felt colder than the bag with air or feathers, leading to the conclusion that heat transfers faster through fat than via an equally thick layer of stationary air. Feathers insulate because they trap air, so there was probably little difference between how the bag of feathers and the bag of air felt.

People in arctic climates use animal skin and fur as well as synthetic clothes to protect themselves from the cold. Synthetic clothes made to withstand extreme weather use the same heat-trapping techniques as the ones found in animals. For example, they might use down feathers to trap air or have a water-resistant coating to keep out moisture.

So long as you used a clean bag, you can reuse the butter for cooking. Also reuse your feathers and other bags as you can.

More to explore
Cool Facts about Heat, from Beyond Penguins and Polar Bears at The Ohio State University
How Animals Stay Warm with Blubber, from Scientific American
Stay Warm with Thermal Insulation, from Scientific American
New Research Reveals How Polar Bears Stay Warm, from Inside Science

This activity brought to you in partnership with Science Buddies

Science Buddies