“That wife of mine is a liar.” said the angry husband to a sympathetic pal seated next to him in the bar.  

“How do you know?” the friend asked.  

“She didn’t come home last night, and when I asked her where she’d been, she said she’d spent the night with her sister, Shirley.”  

“So?” the friend replied.  

“So, she’s a liar. I spent the night with her sister Shirley!”