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Why did #milkgate happen after the Manchester derby?
Sunday's 2-1 win was a massive result for Manchester City, as they beat their biggest rivals at home.
It also set a new Premier League record (the first English team to win 14 games in a row in a season!), and gave them 11 points clear at the top.
But it's what happened after the game that everyone is talking about.
United boss Jose Mourinho had milk thrown at him, and City coach Mike Arteta was left with a cut on his head.
So what happened? The Football Association is now speaking to both sides to try and find out.
The referee, Michael Oliver, didn't see the post-match antics, and so didn't include anything about them in his match report.
But accounts from other people in the area, give some idea of what happened.
It began after City's players finished celebrating in front of their fans after the final whistle.
Players and staff headed down the tunnel to their dressing room, and Jose Mourinho got annoyed at City's post-match celebrations.
He apparently had milk and water thrown at him, while City coach Mikel Arteta suffered a cut head after he was hit by a plastic drinks bottle.
It isn't known who threw the milk or plastic bottle, and both clubs say no punches were thrown during the incident.
Both club's managers have now spoken out about the incident, but without really giving any more information about what actually happened.
Manchester City boss Pep Guardiola says he "encouraged" his side to celebrate Sunday's 2-1 win at Manchester United, but denies they went over the top.
He said: "We were happy. We won a derby against Manchester United, one of the best rivals we have."
He confirmed that City will make a statement to the FA, saying: "After what happened, hopefully it cannot happen again. "
"What happened we will explain to the FA investigation. United will do the same."
Meanwhile Jose Mourinho, appeared at a news conference before Wednesday's game against Bournemouth.
But he didn't want to talk about the milk incident, or what his rival manager had said about it.
This is what he did say: "The only thing I can say is that for me was just a question of diversity, diversity in behaviour, diversity in education, just that and nothing more."
It's not the first time there's been a big food-fight in Old Trafford. Have you ever heard of pizzagate?
It happened back in 2004 in the players tunnel, after Manchester United beat Arsenal 2-0.
As players and officials from both sides clashed, United manager Alex Ferguson was hit by a slice of pizza thrown by an Arsenal player.
English newspapers started calling it The Battle of the Buffet.
But while the FA called both teams to a meeting after that game, and fines and match bans were given out, this was because of action on the pitch rather than the food-throwing itself.
So what will happen this time around?
Man United and Man City both have until 6pm on 13 December to come back to the Football Association with their response.
The FA will then decide if they want to take action, and what kind of action it will be.