"Sunbathing" joke

Joan, the well proportioned secretary, spent almost all of her
vacation sunbathing on the roof of her hotel. She wore a bathing suit
the first day, but on the second, she decided that no one could see
her way up there, and she slipped out of it for an overall tan.
She'd hardly begun when she heard someone running up the stairs. She
was lying on her stomach, so she just pulled a towel over her rear.
"Excuse me, miss," said the flustered assistant manager of the
hotel, out of breath from running up the stairs. " The Hilton
doesn't mind your sunbathing on the roof, but we'd appreciate your
wearing a bathing suit as you did yesterday."
"What difference does it make," Joan asked rather calmly. " No one
can see me up here, and besides, I'm covered with a towel."
"Not exactly," said the embarrassed gentleman. "You're lying on the
dining room skylight."