Whistle at 'em

Murphy and his wife went for a stroll in the park. They sat down on a bench
to rest for awhile.

Soon after, they overhear voices coming from a secluded spot nearby.
Suddenly, Mrs. Murphy realizes that a young man is about to propose. Not
wanting to be eavesdropping during such an intimate moment, she gently
nudges her husband and whispers, "Whistle to let that young couple know that
someone can hear them."

To which Murphy replies, "Whistle? Why should I whistle? Nobody whistled to
warn me!"