Telling lies

"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.

Spoiler:

I spent the night with her sister Shirley!"