Joe drove a bus for a living, but he could never finish his route right on time.

After he finished his route, he would always have to explain to his supervisor why he was early or late. "I was stuck behind a big truck," he explained one day, after arriving ten minutes late.

Thinking that the bus driver was not taking his job seriously enough, the supervisor pressed him on the issue. "But Joe, yesterday you were ten minutes early." reminded his boss.

"Yeah," said Joe, "But yesterday, I was stuck behind a Porsche."