It's a question that has perplexed defences across Europe for the past 13 years.
And there are 600 reasons why nobody has truly found an answer.
How do you stop Lionel Messi ?
Liverpool thought they'd done well enough against the Barcelona talisman in Camp Nou last week, only for the Argentine to burst into life, first by reacting quickest to a Luis Suarez shot that struck the crossbar to put the home side 2-0 up, and then curling in a brilliant free-kick for the landmark 600th goal of his career.
Scoring three goals against the Spaniards will be difficult enough for the Reds in their Champions League semi-final second leg at Anfield on Tuesday.
That, though, must be done while also keeping out Messi, Suarez and the rest.
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“Obviously he is a world class player, probably the best in the world,” says Trent Alexander-Arnold of Messi.
“It will be a really tough battle but in fairness to the lads, we kept him quiet for maybe 80, 88 minutes and he has still produced something special.
“It shows you have to concentrate for the full 90 against him especially. It is about having that respect but not too much respect.
“We can't be scared to look silly against him and try to win the ball back as soon as possible and stop him and other players in their team from hurting us.”
Sometimes there appears little that can be done.
Witness the response by Jurgen Klopp to what he said to Messi when embracing the 31-year-old after the final whistle last week.
“I don’t know if I said something,” says the Reds boss. "You think in these moments, you can’t imagine.
“It’s better that I didn’t say ‘Why did you do that?!’”
Alexander-Arnold was a surprise absentee in Camp Nou – Joe Gomez coming in at right-back for his first start since December – but is expected to start at Anfield.
And that would most likely put him up against former Liverpool team-mate Philippe Coutinho .
But would it be a help to have previously trained with the Brazilian?
“Yeah I suppose,” says Alexander-Arnold. “He is a really tricky player, he can go both sides but it is important not to give those type of players too much space and too much respect.
“That's when they probably hurt you the most, when you stand off them and they pick passes and are able to take you on.
“It will be important to suffocate their players and get the ball back as high as possible and give them as little service as possible.
“The main thing for us is to attack with everything we have got and keep the ball as high up the pitch as possible and win it as soon as possible.
“It is a team full of world-class players and it will be a battle but I am sure we will be able to match them.”
