How is it possible to become your own grandfather?

There is a wonderful explanation in a well-known comic song.

https://en.wikipedia.org/wiki/I%27m_My_Own_Grandpa

That song was featured in The Movie 'The Stupids'

See 'I am my own grandfather!' by Mark Twain at
http://www.natemaas.com/2010/11/more-musical-genealogy.html

My own Grandfather by Mark Twain

After long years as a bachelor I was tired of being alone and married a widow with a grown daughter. My father fell in love with the daughter and took her as his wife. This made me my own son-in-law and my stepdaughter became my mother. After a year my wife gave birth to a son. Now, my son was my father's brother-in-law and at the same time my uncle, since he was my stepmother's brother. But my father's wife also gave birth to a son. So this was my brother and also my grandson, since he was the son of my daughter. This meant I'd married my grandmother, since she was the mother of my mother. As my wife's husband, I was also her grandson. And since the husband of a grandmother is always a grandfather, I am my own grandfather.

A proof that a man may be his own Grandfather.—There was a widow and her daughter-in-law, and a man and his son. The widow married the son, and the daughter the old man; the widow was, therefore, mother to her husband's father, consequently grandmother to her own husband. They had a son, to whom she was great-grandmother; now, as the son of a great-grandmother must be either a grandfather or great-uncle, this boy was therefore his own grandfather. N. B. This was actually the case with a boy at a school in Norwich.