"I'm my own grandpaw", baby
The cover of this sheet music for the 1947 novelty song "I'm my own grandpaw", written by Dwight Latham and Moe Jaffe, contains an old man / baby reversible optical illusion.
"I'm my own grandpaw", old man
The song explains the riddle of how a man can be his own grandfather. It's an old puzzle:
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.I gather that it all makes some sort of logical sense... though not to me. I'm the one who's always nodding knowingly at family gatherings when unknown relatives are introduced, because I get confused with any relations beyond my immediate family.
The Muppets sang about this, which makes it funnier, though not any clearer to me.
I'm my own grandpa