That's Once.

A couple was celebrating their golden wedding anniversary. Their domestic
tranquility had long been the talk of the town. A local newspaper reporter wanted to know the secret of their long and happy marriage.

"Well, it dates back to our honeymoon," explained the man. "We visited
the Grand Canyon and took a trip down to the bottom of the canyon by pack of
mule.

We hadn't gone too far when my wife's mule stumbled. My wife quietly
said, "That's once."

We proceeded a little further and the mule stumbled again.
Once more my wife quietly said, "That's twice."

We hadn't gone a half-mile when the mule stumbled the third time. My wife quietly removed a revolver from her purse and shot the mule dead.

I started an angry protest over her treatment of
the mule, when she looked at me and quietly said, "That's once."

And we lived happily ever after."