Biting, butting, spitting... what is football's deadliest sin?
Fighting and biting are bad but never use an elbow. Headbutting is for cowards. No-one pushes the referee. And whatever you do, do not spit at an opponent.
The incident at St James' Park on Wednesday, in which Newcastle's Papiss Cisse and Manchester United's Jonny Evans appeared to spit at each other, has again highlighted the limits of acceptable behaviour on a football field.
BBC Sport - with the help of pundit Steve Claridge, who played for 22 clubs in a 29-year professional career - examines just how far you can go before you commit the sport's ultimate sin.
Verdict: 'You just don't do it'
Footballers spitting is not an unfamiliar sight. Rarely a minute of a game goes by in which you cannot see a footballer depositing various levels of phlegm on the pitch.
Footballers spitting at each other is less common, though. It not only shocks but also disgusts. The image of Netherlands midfielder Frank Rijkaard spitting into Rudi Voller's mullet twice at the 1990 World Cup is one of the most revolting moments in World Cup history. But why do players dislike it so much?
"It was and is the unwritten rule that you don't do it," said Claridge. "You don't lower yourself to that level. It's not even a football thing; in general life, you just don't do it.
"It's not behaviour we ever want to see, but again, compared to physical harm, it's something players can cope with."
|Football's most notorious spitters|
|Netherlands player Frank Rijkaard spat into Rudi Voller's hair as the West German ran past him after both men had been sent off in a match at the World Cup in Italy in 1990.|
|El Hadji Diouf was fined two weeks' wages by Liverpool, £5,000 by the Glasgow Sheriff Court and given a two-match ban after spitting at a Celtic fan in the front row of the stand during a Uefa Cup tie at Celtic Park in 2003.|
|Arsenal's Patrick Vieira was handed a four-match ban and a £30,000 fine for spitting at West Ham's Neil Ruddock in a Premiership match at Upton Park in September 1999.|
|France and ex-Manchester United goalkeeper Fabian Barthez was given a six-month ban (suspended for three months) for spitting at a Moroccan referee during a friendly between Marseille and Wydad Casablanca in 2005.|
|Alexander Frei of Switzerland was charged with misconduct and banned for three games after a crafty spit on the neck of Steven Gerrard during England's 3-0 win at Euro 2004.|
Verdict: 'Not a big deal'
The term 'fight' in football should be used loosely. Very rarely do we see a scrap worthy of Madison Square Garden on the pitch, although one or two players have certainly shown they pack a tasty right-hook.
Francis Lee and Norman Hunter threw thunderous punches at one another when Derby met Leeds in 1975, and Hristo Stoichkov was initially given a lifetime ban for his role in a ding-dong during the 1985 Bulgarian Cup final.
Stoichkov, who later managed Bulgaria, eventually saw his penalty reduced to a month.
|Steve Claridge on brawling...|
|"It happened to me twice and both times I got sent off for reacting..."|
QPR midfielder Joey Barton, meanwhile, served a 12-match ban for sparking a melee on the final day of the 2011-12 season at Manchester City. Barton was sent off and thrust a knee into Sergio Aguero's thigh before leaving the field. Cue chaos.
"The actual brawl is not what the incident is about," said BBC Radio 5 live pundit Claridge. "It comes after something. It's not a big deal in my view and part and parcel of a physical game.
"I can remember playing for Weymouth and good old Vinnie Jones was involved in a fight with our goalkeeper and it got very unsavoury. But these things happen and once it's done, it's done."
Verdict: 'It can do serious damage'
Mauro Tassotti played seven times for Italy and was given an eight-match ban in his final appearance.
The defender's vicious elbow to the face of Luis Enrique at the 1994 World Cup bloodied the shirt of the Spanish player, but he escaped immediate punishment from the referee as Italy won 2-1 to reach the semi-finals.
Ex-Manchester City defender Ben Thatcher also escaped without a red card when his elbow on Pedro Mendes in 2006 left the Portsmouth player needing oxygen next to the pitch. Again, football's authorities had the final say, handing Thatcher an eight-match ban.
"You can see an elbow that's thrown and someone who genuinely goes up for a header," said Claridge. "I could tell you which ones are intentional every time.
"It can do some serious damage - extreme damage. If you did it 20 years ago, you'd have reprisals, no doubt about it. You could get the guy back.
"If you see something like that it brings a reaction - that's part of being a team-mate. I remember the British Lions rugby team and they would shout a '99' call [on their 1974 tour of South Africa] when they knew, if they heard it, they had to fight and look after one another."
Pushing the ref
Verdict: 'A real curse at the grassroots'
Paolo Di Canio's shove on referee Paul Alcock during Sheffield Wednesday's game against Arsenal in 1998 was a first in England's top flight.
The Italian's reaction to one of his four red cards in the UK saw him banned for 11 games and left pundits, players and supporters aghast.
Referees at the time said Di Canio's ban was too short, but he returned four months later following a move to West Ham, where he became a fans' favourite.
"It won't hurt the referee and it's more of an insult to the authorities," said Claridge. "It's a problem lower down the scale - a real curse in grassroots football - but at the top level, you know the punishment and it doesn't happen.
"It's a tricky balance, though, because if you don't question the referee and fight your corner you're accused of having no passion, yet if you do, you're a thug. Deep down, you know as a player if you've gone too far."
Verdict: 'It sticks in the gut'
A swipe at the ball on the halfway line is one thing, deciding a fixture and leaving a country's World Cup in tatters is quite another.
Diego Maradona's 'Hand of God' goal in 1986 needs no explanation: he leapt, punched, ran away and celebrated as England scratched their heads and exited the World Cup.
More than two decades later, Luis Suarez caused similar controversy when he clawed a header off Uruguay's goalline in the last-minute of extra time in their 2010 World Cup quarter-final against Ghana.
He was sent off, but celebrated in the tunnel when Asamoah Gyan missed the resulting penalty before Uruguay won a subsequent shootout.
Asked whether he would handball on the goalline to win a trophy, Claridge said: "No. I would push the boundaries but I wouldn't cheat.
"It sticks in the gut. You've been cheated. As much as you'd like to do the guy some damage for cheating you, he has not hurt you in a physical sense. But on an emotional level, it's bad. You just hope that person is punished properly."
Verdict: 'Players develop a thick skin'
Suarez, of course has 'previous'. In December 2011 he was given an eight-match ban for racially abusing Manchester United defender Patrice Evra. Uefa has since brought in a minimum 10-game suspension for any player found guilty.
With Fifa pushing so hard to tackle the issue off the pitch, and players reacting so strongly to being abused, is it the one thing footballers detest the most?
Claridge said: "I think, as bad as it is emotionally, players get abused for years playing football and develop a thick skin and I think players are more concerned about being physically hurt than anything verbal. Most just know it's not something you do.
"I also think there is a perception that racism in football is purely white on black, but it's not. There are so many communities now represented but I can honestly say I haven't seen a lot of racism in football. There may be closet racism, but not on the pitch."
Verdict: 'Other sins are worse'
Diving, simulation, over-exaggeration, lying; call it what you want but in a game of such fine margins, gaining even the slightest advantage can make a huge difference.
At the 2002 World Cup, Rivaldo feigned injury, falling to the ground holding his face after a ball was kicked at his leg by Turkish player Hakan Unsal, who was sent off. The Brazilian forward was fined £5,180
In recent years, there has been a drive to eradicate diving, something Fifa vice-president Jim Boyce has previously said is a "cancer within the game".
Dr Maria Kavussanu, an expert in morality in sport at Birmingham University, said: "We have measured all of what we call anti-social behaviours in football, like pushing and fouling - they are aggressive - and we have found the people who commit them are most likely to be the ones who dive.
"Interestingly, you can see as soon as Suarez did what he did with Chiellini and Ivanovic, he faked an injury.
"I would classify the sins which have an extreme impact on others as worse. Diving is maybe not so bad in that context, but so many children watch, especially the best players, and footballers are role models."
Verdict: 'It's a cowardly act'
France legend Zinedine Zidane's butt on Italy's Marco Materazzi during the 2006 World Cup final is one of the most iconic images of the tournament. So much so, it was immortalised in a bronze statue in Qatar, only to be removed later.
Last season, Newcastle manager Alan Pardew received a seven-match ban for butting Hull player David Meyler, while Portugal defender Pepe was sent off earlier in the World Cup for the same act.
Claridge said: "If someone butted me, then we'd be fighting; it's a cowardly act. If you want to fight someone, you fight someone.
"If you're talking about what Pepe did [for Portugal against Germany in the 2014 World Cup] that kind of head-to-head, it won't do damage. There's a massive difference between butting someone and putting your head into someone, and players are strong enough to deal with the mental side of someone trying to provoke a reaction by doing the latter."
Verdict: 'No doubt it can be reckless'
Eric Cantona kick-started an era of dominance at Manchester United, won four Premier League titles and became a club legend, but any discussion about this complex character invariably includes an infamous kung-fu kick.
The Frenchman leapt over the advertising hoardings at Selhurst Park to kick Crystal Palace fan Matthew Simmons - a man he calls a "hooligan". Cantona said it was for fans who would have liked to do it themselves, but concedes "it was a mistake".
Fast forward 15 years and Netherlands midfielder Nigel de Jong brought the martial-arts move onto the field, connecting with the chest of Spain's Xabi Alonso during the 2010 World Cup final.
De Jong was yellow-carded. Cantona was banned for eight months.
"If the ball is there to be won and you go through someone, if you're honestly going for the ball, you have to do it," Claridge said. "Subconsciously you know if a player has tried to catch you high up. The lines are blurred as it all comes down to that - was the player being honest? There's no doubt a challenge like that can be reckless."
Verdict: 'It has broken the boundary of the body'
It sounds almost unthinkable that in the fast-moving environment of a top-level football match, a player has time to bite an opponent.
Liverpool's Luis Suarez did just that when he tussled with Chelsea's Branislav Ivanovic in 2013, less than three years after the striker bit the shoulder of PSV Eindhoven's Otman Bakkal.
Italy's Giorgio Chiellini became the target of a Suarez bite during the 2014 Fifa World Cup, prompting a four-month ban for the Uruguayan.
"It is not about the act itself, it is the symbol it represents," said Orin Starn, Professor of Cultural Anthropology and History at Duke University in Durham, USA. "It has broken the boundary of the body, which in western culture is a taboo. If you go a long way back it is associated with death and evil.
"We regard the body as an entity in itself and something that should not be violated. A bad tackle may break someone's leg but there is no violation of the body so we find that more acceptable than being bitten."
Barton disagrees and, like Claridge, is more fearful of serious physical harm on the pitch. "All things considered I'd rather receive a bite than a leg-breaking challenge," he tweeted after Suarez's grapple with Chiellini.
"Whilst he should be punished, it is not the end of the world. He's a winner. If that means he occasionally steps over the line between right and wrong, than that's what comes with the territory."
Verdict: 'The ultimate sin'
The reckless, over-the-top, studs-up tackle is banned in football's rule book. But, intentionally or not, it still features in the game.
The act has the potential to end a career in a second, leaving a player's hopes and dreams in tatters.
"That's the number one," said Claridge. "The rest, with all due respect, are nothing in comparison. You may as well give up if you're scared what people will say about you in football, but if you're talking of a player intentionally trying to seriously hurt a player in a tackle, that's the one.
"You can do serious damage to the legs and rob a player of all he has known. It happened to me twice and both times I got sent off for reacting to what had happened to me. Even now when I see someone do something like that I think, 'that player has tried to hurt someone, he should not be playing for six months'."
An earlier version of this article appeared on 26 June 2014.