Theory:

     Grandfather gave Nana a slap across her haunches, and she jerked back, dragging Toto with her. He had fastened on to her long ears with his sharp little teeth.
 
     Toto and Nana never became friends.
 
     A great treat for Toto during cold winter evenings was the large bowl of warm water given him by Grandmother for his bath. He would cunningly test the temperature with his hand, then gradually step into the bath, first one foot, then the other (as he had seen me doing), until he was into the water up to his neck.
Explanation:
 
When grandfather noticed Nana skipping its food, he gave it a slap across the haunches. Nana jerked backwards, pulling Toto with it when Grandfather hit it around the haunches.
 
Through Toto's pointed little teeth, it had held tight to Nana's long ears. Grandfather's expectations never fulfilled as Toto and Nana did not become friends anymore.
 
During the cold winter evenings, a special treat awaited Toto regularly. It was nothing but a large bowl of warm water. Toto loved the big bowl of warm water offered by the grandmother for a bath on cold winter evenings. Still, Toto would check the temperature with his hand, then slowly walk into the bath, first one foot, then the other as it had seen the narrator doing so. Then it will immerse into the warm water until it was up to his neck.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Haunch
The back end of an animal
2.
Cunningly
In a cleverly planned way
3.
Gradually
Slowly
4.
Immerse To plunge or dip into a liquid
5.
FastenTo become firmly fix together
Reference:
National Council of Educational Research and Training (2006). Moments. The Adventures of Toto – Ruskin Bond (pp. 7-10). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.