On Saturday I went to the library and on the way there I met a friend of one of my children, accompanied by her boyfriend.
"Hello, what are you doing here?", I asked as I thought she should have been at uni and it seemed a bit early in the term for coming home for the weekend.
She launched into a detailed description of their plans for the day and I began to think I was not talking to who I thought I was.
But it was only when she said "it's dad's birthday today" I realised that I was talking to C, a contemporary of my elder son, who's just lost her adoptive mother recently to cancer and not A, my daughter's friend who has never known her father (coming from a family of six children, all with different fathers - well apart from the twins, that is).
It occurred to me that it was interesting that both girls look so alike, both are pretty and lively and very intelligent, and they come from such vastly different backgrounds.
Still, luckily I don't think she realised that I thought she was someone else.