"That wife of mine is a liar," said the angry husband to a sympathetic pal seated next to him at 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 that she had spent the night with her sister, Shirley."

"So?"

"So she's a liar. I spent the night with her sister, Shirley."