What is a paragraph?

How do you know “When does a paragraph end?” – one child asked me today. He continued,- I am writing a book and I want to know, how do writers decide when to end a paragraphs. He took one thick book of “bharat ek khoj” and shared loudly-“I know when a paragraph ends – suppose I am writing a book on playing cricket, now I can write one paragraph on how to hold the ball, I can write next paragraph how to throw and so on”.