Barcelona's home ground is a massive visitor attraction but the rubbernecking and the souvenir-hunting is only advisable for those on the stadium tour or in the club superstore.
If any visiting manager or players get sucked into the idea that they're on a jolly against Lionel Messi and his mates, switch off for a moment to stare at them in slack-jawed awe, it quickly ends in tears.
It is not uncommon for the marking to get tighter on Messi in the 90th minute than it's been earlier in any game he plays: that's when opportunistic opposition players start to crowd around him, hoping to be the one with whom he will swop shirts at the final whistle. That can be a pretty demeaning spectacle, never more so than when Bayer Leverkusen players squabbled over who had ownership of Messi's shirt after the clubs met last season. Leverkusen's disgusted sporting director, Rudi Voller, found it so pathetic he ordered that the shirt be auctioned off for charity.
There should not be any doubt that Neil Lennon would do the same if his own players were guilty of similarlyboyish behaviour in Camp Nou tonight. The Celtic manager even talked of banning them from swopping shirts entirely at one point yesterday, before quickly recognising that may be unenforceable. It would have seemed a tad hypocritical, too, given that Lennon's Glasgow home contains framed shirts of Barcelona players Xavi and Luis Enrique, with whom he swopped after facing Barcelona in his own playing days.
"People say 'what's it like to play in the Nou Camp'. Well, you come off and say 'I don't know, I don't remember much about it, I was too busy doing this, that or the other!' It possibly won't be that enjoyable for myself this time. The build up is great, but from here on in it's just total concentration."
Lennon's idea of total concentration means a focus on absolutely nothing but the game until the final whistle, not jockeying to be closer to Barcelona's numerous stars in order to swop shirts. "People do all this 'there he is, look at Xavi' and it's all 'can I have your shirt after the game'. But that's not what it's about. What happened with the Leverkusen players was just nonsense. We will respect Barcelona, but we will have to play aggressively at times as well."
Would he stop his players from swopping shirts? "Yes. Well, I can't really stop them because I won't be in the tunnel when the teams walk out, but I just want them to look straight ahead, not look at the side and try to focus on getting a foothold in the game at some stage."
The message appears to have been understood given that James Forrest yesterday admitted he had little interest in any specific Barcelona player's shirt. Besides, he has fulfilled the role of Camp Nou tourist before. Aged nine, he went on a tour of the stadium with his family. Now, aged 21, any wide-eyed gawping from him or his team-mates is liable to be ruthlessly exploited.
It wasn't hard to sense the energy coming off Lennon yesterday. The Champions League is nourishing this Celtic manager, enabling him to ask questions of himself and his players. He revels in the idea of coming up with a way of stopping this extraordinary Barcelona team and there isn't any doubt that it will be a feather in his cap if he manages it. He isn't coming to the vast, nearly 100,000-seater arena with his eyes closed. His first time ended in a 0-0 draw en route to eliminating Barcelona from the 2004 UEFA Cup, and later a 1-1 draw in the following season's Champions League.
He was no longer a Celtic player when Gordon Strachan's team lost 1-0 there in a 2008 Champions League last-16 tie. It is notable that Celtic conceded only two goals in those three recent Camp Nou visits, although Lennon said he felt the current Barcelona team was superior to those he had faced himself.
"This Barcelona team is two or three gears better than anything I have seen before. I watched them the other night and they were fabulous. People say they have defensive problems but I'm not so sure. Deportivo La Coruna's goals on Saturday [in a 5-4 Barcelona win] were a penalty, an own goal and two from corners. They were either set plays or a freak of nature. I don't recall [Barcelona goalkeeper] Victor Valdes having many saves to make during the match but Barcelona could have scored six or seven. At 3-0 up after 15 minutes we might as well have got up and left . . . so looking at it, we know we are in for a really tough examination. Talking to people in the game, what Barcelona have done as a club has come from Holland.
"They have taken 30 years to get it right at the academy level, to bring these younger players through. For a while, until [former coach] Frank Rijkaard came in, there was something missing, from 1992 until 2003 or 2004. But since then they have been by far the best football team on the planet. One of the best I have ever seen.
"They're just brilliant footballers. They're so intelligent. They're strong – believe it or not – they're brave, they cover the ground quickly, they are just the ultimate team at the moment."
Celtic, like any visitor, must simply try to live with Barcelona tonight. Every trick has been tried in the usually fruitless attempt to contain them. Lennon will fill the role of an art lover trying to thwart an exhibition.