Sally and Nancy were going in tube, chatting about their husbands. 
Nancy: "It's unbelievable, I can never figure out where he goes at night."
"I know exactly what you mean," said Sally. "One second he's in the house, and the next he's gone without a trace."
A surprising voice came from a stranger lady sitting next to two ladies, "Well," The stranger lady continued, "I always know where my husband is."
"How do you manage that?" the other two women ask.
"Easy," she replies. "I'm a widow."