Theory:

    “Listen, Tilloo! I work on a surface where an ordinary person cannot survive because the air is too thin to breathe and where the temperature is so low that you would freeze to death.”

    “But, Daddy, how do you manage to survive?”
 
    “Because I go well-equipped, in a special suit with a reservoir of oxygen. I can keep warm and my boots are specially designed to make it easy for me to walk on the surface. What’s more, I’ve been trained to survive and work on the surface.
 
    “There was a time when our forefathers lived on the surface quite naturally. Indeed they did so for millions of years. But times changed. The very sun, which had provided us with sustenance, turned hostile. It changed only slightly but the change was sufficient to upset the balance of nature on this planet.
Explanation:
 
Then Tilloo’s father began to explain to him the nature of his job and its dangers. Later his father stated that he was working on a planet's surface where ordinary people cannot survive. For instance, people need oxygen to breathe, but it wasn't easy to survive on the planet’s surface because the air was very thin to breathe. Also, the temperature in that place seemed to be very low. It could freeze a person to death. Moreover, the person would die if they lived on the surface of the planet without any proper precautionary measures.

While hearing the dangerous statement from his father, Tilloo questioned how his father managed to survive on the surface of the planet. Later his father stated that he would go to the planet's surface using well-equipped suits with a reservoir of oxygen. Also, the suit would keep him warm, and he used specially designed boots that would help him to walk easily on the surface of the planet. In addition, he stated that he had been trained well to survive and work on the planet's surface. He mentioned the above things to his son to understand how difficult it was to stay on the planet’s surface.
 
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A man wearing special suit with a reservoir of oxygen

Furthermore, Tilloo’s father stated that his forefathers had formerly lived organically on the planet’s surface without getting any harm. It meant that his forefathers lived on the surface of planet quite naturally. They lived for millions of years. The times, however, had changed. The sun, which had previously provided them with food, had turned against them. It indicated that his forefathers had lived contentedly on the surface, unharmed, but that the sun had become harsher over time, and whatever nourishment it had offered them had become wild. As a result, living on the surface was dangerous. In addition, Tilloo’s father stated that there was only a small change in the sun, yet it was enough to upset the planet’s natural balance. And so, Tilloo and his family were living beneath the surface of the planet.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Survive Continue to live or exist, in spite of danger or hardship
2
Freeze A period of frost or very cold weather
3
Temperature The degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a thermometer or perceived by touch
4
Reservoir A receptacle or part of a machine designed to hold fluid or oxygen
5
ForefatherA member of the past generations of one's family or people; an ancestor
6
SustenanceFood and drink regarded as a source of strength; nourishment
7
HostileUnfriendly and not liking something
Reference:
National Council of Educational Research and Training (2007). Supplementary. An Alien Hand - Jayant Narlikar (66-73). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.