Question: What has David Luiz done for Chelsea that John Terry never has, and probably never will?
The answer is he played in, and won, a Champions League final.
That riddle, I suspect, is behind the most puzzling transfers of this summer's window in Europe. The return of David Luiz from Paris Saint-Germain for £32.73 million (S$59 million) surprised even Chelsea's new manager Antonio Conte, and it happened too late on deadline day to count until now.
Whether the Brazilian's distinctive floppy hair will be seen at Swansea today when Chelsea resume their season after the international break will depend on Conte's conclusions from the precious little training time that David Luiz and the other late signing Marcos Alonso gained with Chelsea's players flying home from representing their countries across the globe.
Conte's Friday press conference was monopolised by the David Luiz question.
The media asked whether Chelsea had panicked in going back into the market to pay big money for the defender who many consider as a self destruct button?
"I am happy David Luiz is with us because I know that together with Marcos Alonso he can improve a lot this squad," the new manager responded.
Chelsea soaked up Bayern attacks for 90 minutes, then for 30 minutes of extra time. The Brazilian with a hole in his hamstring (and a permanent deformity in that thigh) kept his concentration and, when his turn came, scored in the penalty shoot-out.
The critics, led by ex-pros who always know better, persisted.
Conte blinked, but stood his ground. In his surprisingly good English given that it is only a few months since he left Italy, he said: "I am a bit surprised about this situation around David Luiz because I think he is a good player. He showed this in his career because he has won a lot.
"In some situations, he lost. But I think he is a good defender - a good centre-back."
The naysayers do not trust David Luiz at centre-back. They see him, at best, as a defensive midfielder, someone whose natural instinct to counter-attack hides lapses in concentration.
"I don't speak about the past," Conte answered. "I am now coach of David Luiz and I have a great responsibility to improve him. If someone thinks a player loses concentration during the game, my task is to improve this aspect and I am not afraid of this."
I like Conte. I like his upfront honesty as much as I like watching him pour his heart and soul out from the touchline.
A coach who meets criticism head on, and who talks about sharing responsibility, is a big improvement on Jose Mourinho's proclivity to take the praise when his teams win, and to point the finger at players when they lose.
That is why Conte is at Chelsea now (and good luck to the Manchester United stars when that sours too).
But David Luiz certainly divides opinions. A big man, 1.89 metres, he was sold to PSG for a record fee for a defender (reported at anywhere between £42.08 million and £50 million) after Mourinho returned to the Bridge three years ago.
David Luiz insists there was no ill feeling between them, but he wanted the challenge in Paris.
Footballers rarely speak ill of former work colleagues. The carousel of players and managers changing clubs, especially at this level, might well come around and throw them together again.
What we know is that Mourinho chose not to pick David Luiz, and the Brazilian chose to leave.
But one man above both of them has admiration, and gratitude, for the Brazilian.
On the eve of the 2012 Champions League final between Bayern Munich and Chelsea - in Munich - Chelsea's owner Roman Abramovich was a nervous man around the team.
He knew that John Terry and Branislav Ivanovic had ruled themselves out of the final, banned for foul play. Abramovich was told that David Luiz had a torn hamstring.
The player told the interim manager Roberto di Matteo and the medical staff that, injured or not, he wanted to play. He told the owner the same thing.
This is a side of the player we don't see. The widely-held impression is that Luiz is too cavalier. He has personality, energy, and is far more technically equipped than the average centre half.
But having to depend upon him when you must defend for your life against the likes of Thomas Muller and Arjen Robben? "No problem boss, I play," David Luiz said to both di Matteo and Abramovich.
He played. Chelsea soaked up Bayern attacks for 90 minutes, then for 30 minutes of extra time. The Brazilian with a hole in his hamstring (and a permanent deformity in that thigh) kept his concentration and, when his turn came, scored in the penalty shoot-out.
Chelsea won because players like him kept their nerve, whereas Robben for example, one of the hottest shots in football, missed his penalty kick.
Fast forward to 2014 when, with Muller rampant and Luiz all over the place, Brazil suffered the biggest humiliation in World Cup history. Germany won 7-1, in Brazil, and Luiz wore the captain's armband that day.
But he came back. He won the French championship with PSG just as he had won trophies with Benfica and Chelsea.
And now, doubtless with Abramovich the paymaster having the final say, he is back at Chelsea: As a defender, not as a midfielder according to Conte.
"I want to work with him, and with my players," Conte insists. "It is important to remember that when I started at Juventus as coach with (Leonardo) Bonucci, (Giorgio) Chiellini and (Andrea) Barzagli, the defence conceded a lot of goals.
"Many people told me they are not good, these defenders. I started to work with them and found a great commitment from these players - and through the work, they are now talked about as three great defenders in the world.
"I think David Luiz's qualities are somewhere in the middle between Bonucci and Barzagli. He likes to play football like Bonucci, and he has good physical potential like Barzagli."
Can a leopard change his spots? David Luiz is 29. With Conte as his teacher, he might yet grow into that leader of the defence that Terry cannot forever be.