How do you get to lead 41-0 at halftime?

First, you need to have the best defense in the league with the best turnover margin in the league (and that even with the abysmal performance by Rex Grosman against Arizona).

Second, you need to play against second worst defense in the league with the worst turnover margin in the league (having four fumbles recovered never hurts...)

I would add that you have to have the best offense in the league, but apparently this is totally unnecessary against 49ers :)