Theory:

     Velu was puzzled. He had heard of people throwing away rubbish. But why would anyone want to buy rubbish?

     “Who’s Jam Bazaar Jaggu? Why’s he buying all this?”

     “You think he buys it for show? He sells it to a factory. Come on, I don’t have time to waste, like you.”
 
     Velu did not move. He hadn’t run away and come to this new place to dig through garbage bins. Jaya poked at him with her stick.

     “Look here!” she shouted. “If someone gets there before us we don’t get anything. Don’t just stand there, posing. Big hero. I’m trying to help you. Who filled your stomach today?”

     Velu scratched his head and sighed. I’ll do it for now, he thought, until I find a better job.
Explanation:
 
Velu was confused, as he had never heard of people collecting garbage and selling it to the shop. Velu was puzzled, and he was thinking about what they will do with the leftovers taken from the rubbish. When Velu was in his village, his people would throw  garbage into the dust bin, and they would never sell it to anyone. Velu was interested in knowing about Jam Bazaar Jaggu and why he was paying money for the garbage.

Velu thought that usually, people would buy expensive things for their happiness, but why a person like Jaggu is getting garbage and paying money for it. To clear his doubts, Jaya explained to him that Jaggu was not buying it for entertainment. Jaggu would sell the plastics, glass and other things to a factory. From the factory, the items would go for recycling. Recycling is a process of using things again and again or making new things out of used ones. It is always better to recycle than to throw things away. The things we throw away do not disappear, and they pollute the air.

Jaya took the stick and poked him, and she told Velu she did not have time to waste. Other ragpickers would come to collect the garbage, and they had to reach the spot earlier. Jaya told him not to waste time by simply standing at a place; he needs to work hard to earn money and buy good food. Jaya asked him, "Who filled your stomach today?” Velu scratched his head and took a deep breath. Velu thought of continuing as a ragpicker until he finds a new job for him.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
EntertainmentThe action of providing or being provided with amusement or enjoyment
2
Pollute Contaminate (water, the air, etc.) with harmful or poisonous substances
3
SighedEmit a long, deep audible breath expressing sadness, relief, tiredness, or similar
Reference:
National Council of Educational Research and Training (2008). It so happened. Children at work- Gita Wolf, Anushka Ravishankar and Orijit Sen (pp. 7-16). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.