Son & Dad

Son : Daddy, I fell in love & want to date this awesome girl.
Father : That's great son. Who is she?
Son : It's Sandra, the neighbour'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 neighbour's daughter.
Father : Ohhhh I wish you hadn't said that. Angela is also your sister. This went on couple of times and son was so mad, He went straight to his mother crying.
Son : Mum I am so mad at dad! I fell in love with six girls but I can't date any of them because dad is their father!
The mother hugs him affectionately and says : My love, You can date whoever you want. He isn't your Father..!!