The doc told a guy that masturbating before sex often helped men last longer during the act.

The man decided, "What the hell, I'll try it," He spent the rest of the day thinking about where to do it before he got home to his wife. He couldn't do it in his office. He thought about the restroom, but that was too open. He considered an alley, but figured that was too unsafe. Finally, he realized his solution. On his way home, he pulled his truck over on the side of the highway. He got out and crawled underneath as if he was examining the truck. Satisfied with the privacy, he undid his pants and started to play with his unit. He closed his eyes and thought of his lover. As he grew closer to the big finish, he felt a quick tug at the bottom of his pants. Not wanting to lose his mental fantasy or the orgasm, he kept his eyes shut and replied, "What?" He heard, "This is the police. What's going on down there?" The man replied, "I'm checking out the rear axle, it's busted." The cop replied, "Well, you might as well check your brakes too while you're down there because your truck rolled down the hill 5 minutes ago."