Your child

Bob had never intended to remain a bachelor all his life. However, he enjoyed the liberty of a life on his own, so he postponed Hymen's bands as long as possible. He was now in his early forties and more and more felt like the time was ripe. 

One morning, the doorbell rang. He slouched to the door barefoot, past the piles of unwashed t-shirts and posters of Che Guevara and Bob Dylan. 

When he opened, there was a stunningly beautiful woman standing outside the door.

"Mr Bob?" she asked. "I am ms Mary from the social department. I come here with your child."

Behind her, there was a small boy in dirty clothes and dishevelled hair. 

"There must be some mistake. I do not have any children." 
"I did not say you knew him, but it is your child. Sometimes, mr Bob, men get children without knowing it, you know."
"I assure you that I keep track of such things. I may sleep around, but i know with whom, and that is not my son."
"And you always kept track of all women you slept with?"
"Ever since I was a teenager."
"I believe you,  mr Bob. Nevertheless, this is your child. Do you remember that school trip to Paris, when you were fifteen? You did not keep as good track of things at the time, did you? One of the girls you met became pregnant."
"Even if that would be true, that was 25 years ago. That child is no older than five. No way that is my son."
"I'm four", whispered the child. 
"I never said this was your son, mr Bob. Your son and his wife died in a car accident two weeks ago. This is your grandson. As only surviving relative, you get custody. Goodbye. Be nice to granddad, Eric."