Knitting Jokes And Puns

I heard this joke probably 35-40 years ago and just thought of it again recently. My dad loved this joke, but I can't remember the punchline.

An older couple had been married for many, many years. So many years in fact that she knew all his jokes my heart. They decided to save time by numbering all of his jokes. While sitting together in their rockers, he would lean over and whisper, "Number 7." She giggled while continuing her knitting. A few minutes later, he learned over again and whispered, "Number 12." This time, she laughed out loud.

Then something funny happens.