Theory:

     Margie did so with a sigh. She was thinking about the old schools they had when her grandfather's grandfather was a little boy. All the kids from the whole neighbourhood came, laughing and shouting in the schoolyard, sitting together in the schoolroom, going home together at the end of the day. They learned the same things, so they could help one another with the homework and talk about it.
 
     And the teachers were people…
 
     The mechanical teacher was flashing on the screen: “When we add fractions ½ and ¼...”
 
     Margie was thinking about how the kids must have loved it in the old days. She was thinking about the fun they had.
Explanation:
  
Margie’s exhaled heavily as her life was mechanical, just like a machine. There is no fun in her classroom. Margie is thinking of the story said to her by her grandfather. Her grandfather told her that his grandfather used to go to school when he was a little boy. She remembered that life. When Margie comes to know that in the past, children used to go to school, they sat together, enjoyed in the school, she gets curious about it and wants to go to school. Margie feels that in school, they get the same homework, and they can discuss it, take help as well. So, all the students become friends and study in a fun way.
 
She found it very strange that the teachers were human beings who can understand their emotions as well.
 
As Margie had inserted her homework, the machine started with the lesson.
 
The author ends the story with the title of the chapter ‘The Fun They Had’. (The fun the students had when they went to real schools). Margie is just pondering about the kids of the past who went to school and enjoyed studies.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. The fun they had - Isaac Asimov (pp. 150-153). Published by the Tamil Nadu Textbook and Educational Services Corporation.