"The Model Lodger" joke

Doris and Fred had started their retirement years and decided to raise some extra cash by advertising for a lodger in their terrace house.
After a few days, a young attractive woman applied for the room and explained that she was a model working in a near-by city center studio for a few weeks and that she would like the room from Mondays to Thursdays, but would pay for the whole week. Doris showed her the house and they agreed to start straight away.
"There's just one problem," explained the model. "Because of my job, I have to have a bath every night, and I notice you don't have a bath."
"That's not a problem," replied Doris. "We have a tin bath out in the yard and we bring it into the living room in front of the fire and fill it with hot water."
"What about your husband? asked the model. "Oh, he plays darts most weekdays, so he will be out in the evenings," replied Doris.
"Good," said the model. "Now that that's been settled, I'll go to the studio and see you tonight."
That evening, Fred dutifully went to his darts match while Doris prepared the bath for the model. After stripping off, the model stepped into the bath. Doris was amazed to see that she had no pubic hair. The model noticed Doris' staring eyes, so she smiled and explained that it is part of her job to shave herself, especially when modeling swimmer or underclothes.
Later when Fred returned, Doris related this oddity and he does not believe her. "It's true, I tell you!" said Doris. "Look, if you don't believe me, tomorrow night I'll leave the curtains slightly open and you can peek in and see for yourself."
The next night, Fred left as usual and Doris prepared the bath for the model. As the model stepped naked into the bath, Doris stood behind her. Doris looked towards the curtains and pointed towards the model's naked pubic area. Then she lifted up her skirt and wearing no panties, pointed to her own hairy mass.
Later Fred returned and they retired to bed. "Well, do you believe me now?" she asked Fred.
"Yes, he replied. "I've never seen anything like it in my life. But why did you lift up your skirt and show yourself?"
"Just to show you the difference," answered Doris. "But I guess you've seen me millions of times."
"Yes, said Fred, "I have - but the rest of the dart team hadn't."