"Playing Cards" joke

Little Johnny woke up in the middle of the night and went to the bathroom. On the way back to bed, he passed his parents room.

When he looked in, he noticed the covers bouncing. He called to his dad, "Hey Dad, what are you doing?"
The dad answered, "Playing Cards".
Little Johnny asked, "Whose your partner?"
The dad answered, "Your mom".
Little Johnny then passed by his older sister's room. Again, he noticed the covers bouncing. He called to his sister, "Hey Sis, what are you doing?"
The sister answered, "Playing Cards."
Little Johnny asked, "Whose your partner?"
She answered, "My boyfriend."
A little later, the Dad got up and went to the bathroom.
As he passed Little Johnny's room, he noticed the covers bouncing. He called to his son, "What are you doing?"
Little Johnny answered, "Playing Cards."
The Dad asked, "Really? Whose your partner?"
Little Johnny answered,... "You don't need a partner if you have a good hand!"