Manchester City's victory over Manchester United sends out a massive message to everyone. That's probably the best they have been going forward since Roberto Mancini has been in charge - and what a day to do it.
The 6-1 thrashing at Old Trafford says more about City than it does about their closest rivals and makes them an even bigger scalp - it is the announcement that they have seriously arrived on the big stage.
You lose games in the Premier League and lick your wounds and go on to the next one, but City just absolutely clicked. They won't have too many days like that so this was a very special day for them.
Mancini picked his team to win the game, which made total sense because they have got such good players. The likes of Mario Balotelli and David Silva have to go to places like United and take them on.
No-one has ever doubted Balotelli's ability and he had a fantastic day. If you look at his record at City when coming on from the bench his goals ratio is very good. Mancini has just got to get him on the pitch and keep him on the pitch.
But one swallow does not make a summer, so let's wait until the end of December and see in how many more games he looks like becoming one of the top three players in the world - as Mancini has claimed.
For United, this has actually been coming for a while. Not the 6-1, obviously, which is a massive shock, but Liverpool created quite a few chances against them in the 1-1 draw last week and Basel did the same in the 3-3 draw at Old Trafford in September.
Chelsea also created lots of chances against them earlier in the season and they have looked very vulnerable defensively.
United are easier to get at at the moment. Anderson, while offering something a bit different in midfield, is defensively a long way short of the job Paul Scholes used to perform in front of the back four. People forget the job Scholes did.
The Manchester United midfielders used to be a great barrier in front of the defenders but at the moment they are very easy to run at and create chances against, which is most unusual.
They've lost 6-1 which is seismic, but they've only lost three points. At moments like this in football everyone says City are going to win the league easily - but it doesn't work like that, as we know.
United boss Sir Alex Ferguson will get the tin hat on, start the 'we're not favourites to win' line and they will now start just plugging away at City. The goal difference is almost like an extra point.
In the Premier League, we tend to see a massive over-reaction when teams win or lose - managers are absolutely brilliant or they're hopeless. We wait and see because Manchester United are the past masters at chipping away at teams.
Sunday's results were absolutely amazing. Chelsea had just kept nicely plugging away in terms of the way they play under a new manager who is inexperienced in the Premier League.
They were winning games but not attracting big headlines, with everyone saying the title is going to one of the Manchester clubs, so their defeat was as much of a shock to me as City winning at United.
Mark Lawrenson was talking to BBC Sport's Paul Fletcher.