简单的英语笑话:The first time, the second time, the third time

The first time, the second time, the third time
　　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 lady. We visited the Grand Canyon and took a trip down to the bottom of the canyon by pack mule. We hadn't gone too far when my husband's mule stumbled.
　　My husband quietly said 'That's once.' We proceeded a little farther when the mule stumbled again. Once more my husband quietly said, 'That's twice.' We hadn't gone a half-mile when the mule stumbled a third time.
　　My husband promptly removed a revolver* from his pocket and shot him.
　　I started to protest over his treatment of the mule when he looked at me and quietly said 'That's once.'