English joke : You married my mother

A six-year-old boy walked up to his father one day and announced, 'Daddy, I'd like to get married.'
His father replied hesitantly, 'Sure, son, do you have anyone special in mind?'

'Yes,' answered the boy. 'I want to marry Grandma.'

'Now, wait a minute,' said his father. 'You don't think I'd let you get married with my mother, do you?'

'Why not?' the boy asked. 'You married mine.'