Theory:

    When Ranji saw the pool, he did not hesitate to get into it. He had often gone swimming, alone or with friends, when he had lived with his parents in a thirsty town in the middle of the Rajputana desert. There, he had known only sticky, muddy pools, where buffaloes wallowed and women washed clothes. He had never seen a pool like this — so clean and cold and inviting. He leapt into the water. His limbs were supple, free of any fat, and his dark body glistened in patches of sunlit water.
Explanation:
 
The narrator then stated that Ranji couldn't stop himself from diving into the pool after seeing it. When he encountered the pool, he thought of jumping into it.
 
swimming pool.png
Ranji jumped into the pool

Later the narrator revealed to his readers that Ranji came from the "Rajputana desert". Ranji lived in the middle of the Rajputana desert with his parents. While he was living there, he frequently went swimming, either with his friends or alone. From there, one can understand, Ranji would do things on his own, without the assistance of others. Furthermore, the narrator described the dwelling place of Ranji as a "thirsty town." His statement was based on the fact that it was a desert and there would be an insufficient supply of water. As a result, it was referred to as a parched town with a scarcity of pure water.

In addition, the narrator mentioned that Ranji had only seen sticky pools filled with mud in his town. Furthermore, buffaloes used to roll about or lie relaxed in the pool, whereas women used to wash their clothes in it. The pool appeared to be sticky as a result of this. Ranji was reminded of his hometown's muddy water pool upon witnessing the pool with clear water. He had never seen such clean pools anywhere else.
 
dirt.png
Muddy pool
 
The narrator also tried to convey to his readers that the pools in the middle of the Rajputana desert had become filthy due to the use of laundry and buffaloes. The pool in the forest, on the other hand, would be pure because it was not utilised for such reasons, and the water flows directly from the cluster of rocks. It was the reason behind the pool in the forest looked neat and clear.

While seeing the clear pool, Ranji was attracted to it because of its cleanliness and coolness. As a result of it, he jumped into the water. After diving into the pool, Ranji felt that his legs were moving flexibly in the cool water. Also, the narrator mentioned that as he was swimming freely on the pool's surface, his limbs were free of fat. In addition, the term "free of any fat" implies that as Ranji was a thin boy who did not have any fat on his body and swimming was meant to be easy for him. However, when he swam in the pool, his brown body glowed in the sunlit water. It implies that his dark body seemed to be shinning when the sunlight fell on the water.

From the above paragraphs, one can easily understand what made Ranji jump into the water without a second thought. He jumped into the water because it was clear and calm, and he wanted to enjoy swimming and get away from the heat of the summer season.
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Hesitate To pause before you do or say something, often because you are uncertain or nervous about it
2
SuppleEnding and moving easily and gracefully; flexible
3
Glisten To shine by reflecting light from a wet or smooth surface
4
Wallow Rolled about in mud or dirty water
Reference:
National Council of Educational Research and Training (2008). It so happened. The Fight- Ruskin Bond (pp. 45-54). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.