Son: “Daddy, I fell in love and want to date this awesome girl.”
Father: “That’s great, son! Who is she?”
Son: “It’s Sandra, the neighbor’s daughter.”
Father: “Ohhh, I wish you hadn’t said that. I have to tell you something, son, but you must promise not to tell your mother. Sandra is actually your sister.”
The boy is naturally bummed out, but a couple of months later:
Son: “Daddy, I fell in love again and she is even hotter!”
Father: “That’s great, son! Who is she?”
Son: “It’s Angela, the other neighbor’s daughter.”
Father: “Ohhh, I wish you hadn’t said that. Angela is also your sister.”