A secret

A couple was celebrating their golden wedding anniversary. Their domestic tranquillity had long been the talk of the town. A local newspaper reporter was inquiring as to 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 on the canyon by pack 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 twice.'
We hadn't gone a half- mile when the mule stumbled the third time. My wife quietly removed a revolver from her pocket and shot the mule dead. I started to protest over her treatment of the mule when she looked at me and quietly said 'That's once.'"