Sweet and subtle, this was posted on the groaners listserv. The author is not known.

Little Noah came into the house with a new harmonica. “Grandpa, do you mind if I play this in here?”

“Of course not, Noah. I love music. In fact, when your grandma and I were young, music saved my life.”

“What happened?”

“Well, it was during the famous Johnstown flood. The dam broke and when the water hit out house it knocked it right off the foundation. Grandma got on the dining room table and floated out safely.”

“How about you?”

“Me? I accompanied her on the piano!”