There is a story at the beginning of this chapter in NCERT book. This story was written by Dr. Zakir Hussain. He was the third President of India.
This story is about a woodcutter and another person, named Mian Balishtiye. The woodcutter is feeling cold. He blows air from his mouth on his hands. This makes his hands warm. When a puzzled Mian Balishtiye asks about it, the woodcutter tells that blowing air on hands makes them feel warmer.
After sometime, the woodcutter lights fire to cook. He blows on firewood to burn them faster. After that, he takes out boiled potatoes to eat. The potatoes are too hot to handle. So, the woodcutter blows on them. This helps in cooling down the potatoes.
Mian Balishtiye is shocked. He thinks that the woodcutter knows some magic because he can light fire by blowing on firewood, and he can cool down potatoes by blowing on them.
Some of the things around you are cold. Some other things around you are hot. Ice cream is very cold, while tea can be hot. But flame on the gas stove is hotter than tea. You can comfortably hold a cup of hot tea. But you cannot touch a hot girdle because it is too hot.
When the woodcutter blows air on his hands on a cold winter day, the hands feel the warmth of air. This happens because the air (blown from mouth) is warmer than the air around.
When the woodcutter blows air on hot potato, they become colder. This happens because the air (from mouth) is colder than potato. You must have blown air over a cup of tea in order to cool it down.
This shows that feeling of warmth or cold is a relative thing. To understand this, you can do an experiment.
How does it feel? Does the water in C feel warmer or colder?
You will notice that the water in C feels warmer to your left hand but feels colder to your right hand. This experiment shows that feeling of warmth or cold is a relative thing. That is why blowing air on hand on a cold day makes the hand feel warmer. On the other hand, blowing air on hot tea makes it colder.
You may have seen your father or a classmate blowing on spectacles to clean them. When you will blow air on a mirror, the mirror becomes foggy. This happens because of moisture in air. When someone blows air on glass, the water vapour in air cools down and turns into liquid water. This creates a very thin layer on glass and makes the glass foggy.
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