When I was very little my mother used to point at the television whenever this actress came on and go "Look! Look! Daddy's girlfriend!"
I had only a vague idea of what girlfriends were, but I decided that my mum was saying that before he met her, my dad had been going out with Eva Gabor. This made me think my dad must have been extremely cool.
I wondered why they weren't still together. In those days, I didn't understand how relationships ended (or even, I suppose, that they could...) I was only five at the time, and my dad was really, really old. I didn't understand about mistresses, either, but I thought it was better not to ask my mum too many questions.
The existential implications of having a different mother didn't even cross my mind.