I'd ordered her a cake, but when I got to the bakery to pick it up, the cake decorator had transposed the digits of her age.
"She'll thank you for the compliment," I said, "but her age is a prime number, and the number on the cake is the product of the digits of her age at not one, but two future birthdays."
"You don't think she'll break any longevity records, then?" the cake decorator shouted after me as I was leaving the shop without my cake.
How old is my sister?