What’s the meaning of the story?

He said, “The last time I took the umbrella, I left it at a restaurant.” She said, “Put a rubber band around your finger to help you remember the umbrella” He said, “The last time I did that, I forgot to remove it and my finger got all puffy. I could’ve lost my finger.” She said, “Tie a string around your thumb. That’ll remind you to remove the rubber band”

Why?!? Why he gonna remember by tying a string around his thumb?

And, what is this meaning? Rubber band around your finger and, in addition, a string? Are the rubber band and a string connected with each other?

A kind of mischievous?

Any information would be appreciated. Thanks in advance.