Willie Nelson has a song explaining this.
So does Ray Stevens
I’mmm my own grandpaaaa, it soundss funny i knoww but it really is sooo, oh I’mmm my own grandpaaa
For those looking for it, it's "I'm My Own Grandpa, which is a cover of The Jester's "I'm My Own Grandpaw".
Writers Dwight Latham and Moe Jaffe were inspired by a short Mark Twain story from the 1889 collection Wit and Humor of the Age:
Very Closely Related
"Well, Sam, I'll tell you how it is. You see, I married a widow, and this widow had a daughter. Then my father, being a widower, married our daughter, so you see my father is my own son-in-law."
"Yes, I see."
"Then again my step-daughter is my step-mother, ain't she? Well, then, her mother is my grandmother, ain't she? I am married to her, ain't I? So that makes me my own grand-father, doesn't it?"
I did the nasty in the pasty.
Futurama is the only correct reference to use here.
Indeed! That past nastification...
This is too complicated. Why not just marry your own grandmother?
Ah, so finally we have an answer to Vethala's last question!
(In Indian mythology, the demon Vethala asks the wise king Vikramaditya several riddles. He answers all but one correctly. The one he cannot answer is this - if a widow and her daughter marry a youth and his widowed father respectively, and each couple has a child, what is the relation between the children?)
It can be confusing without crossing the generational layer. A friend of us married. At the wedding, the bride's (widowed) father met the groom's (also widowed) mother. They married a few months later.
Holup