It all starts with a jump. It does not tell those observing it everything they need to know, but it's not far off. They can fill in the details later using some of the most sophisticated sports science in existence, drawing on biochemistry, neurology and kinesiology, the study of movement. They can seek answers in the "mind room." They can use your teeth to tell about your bones. They can perform all sorts of tests with all sorts of state-of-the-art equipment. But it all starts with a jump.
Football is obsessed with immortality. It is a sport heaving with countless would-be Ponce de Leons all searching for the elixir of eternal youth. Few industries are so resistant to the concept of decline, so reluctant to accept that the clock ticks and the sun sets. It spends hundreds of millions of dollars every year trying to stave off the ravages of time. It does all it can not to have to say goodbye.
AC Milan came closer than most to succeeding in this most quixotic of quests. At Milan Lab, the research centre just south of Lake Varese started by Belgian chiropractor Jean-Pierre Meersseman, they thought they could rinse five, six or seven extra elite years out of their players.
It was there that everything started with a jump. Meersseman has it that the information he could glean from that one action spoke volumes about any player's physical structure, condition and necessary treatment.
He would then put his arsenal of researchers, experts and contraptions to work. He prolonged the careers of some of Milan's golden generation beyond all expectation. Paolo Maldini played until he was 74 (well, 40). He sliced David Beckham's body fat in half in just a few days. He nearly stopped the clock. He nearly held back the night.
Eventually, age overpowered even Meersseman's alchemy. One by one, time caught up with Milan's greats, and one by one, they slipped away. Milan went with them. Never before has a club been so reliant for so long on one group of players. Maldini's generation kept Milan among the elite for almost two decades thanks to the wonders worked at Milan Lab.
When they were gone, the club lost its way. It is seven years since the last remnants of that side won the Champions League for the last time. The club has been rudderless ever since. It sank into midtable obscurity, into a shadow of what it once was. It is not clear how long it will take to see the club rise again. The longer the night before, the more painful the morning after.
Milan are not the only club to harbour this obsession, though, to believe that they alone are immune to aging. It is one of those few unifying forces that cuts across national and tribal allegiance, applied in equal measure to all teams.
It affects fans as well. Supporters do not want to see their heroes leave, so used to casting them as supermen who cannot be tamed that the idea of them growing old and losing their powers is disconcerting anathema.
But more than that, it touches players and managers. Professionals resent being told that one of their number is not what he was; their memories are still strong, their faith unwavering. Those men whose destiny and popularity rely on the graying demigods dare not consider what they might do without them.
Manchester United could not envisage a time without Ryan Giggs and Paul Scholes. Real Madrid never wanted to bid farewell to Raul. Barcelona never deigned to replace Carles Puyol; they will therefore fight to retain Xavi Hernandez for as long as possible. It took an almighty demonstration of willpower for Chelsea to dispense with Frank Lampard and Ashley Cole; that they are not impervious to romance can be seen from their decision to re-sign a declining Didier Drogba, albeit in a lesser role.
Most immediately of all, though, there is Liverpool and Steven Gerrard.
Gerrard is as symbolic to Liverpool as Maldini was to Milan. He has been the cornerstone of the club for 15 years. He has not merely been present, though. He has been the reference point. He is the spirit of Istanbul made flesh. He is the goal against Olympiakos, the screamer in the FA Cup final and the reducer of Old Trafford. Think of a moment that has brought joy to Anfield this century and Gerrard is at its centre. No wonder it is hard to think of a world without him.
Suggesting that it might be time to do so is heresy among many -- though certainly not all -- fans. It is absolutely unfathomable to those who have been his teammates. As far as they are concerned, Gerrard will be able to play until he decides his time is up. That is always the way players see these things, with the power firmly in their own hands.
More significant is that Brendan Rodgers has scotched any hint that his captain might be fading into obsolescence.
"I think," said the Northern Irishman after defeat at West Ham, "if you look at his numbers and games and minutes he has played, you'll see in the last couple of seasons that it is as good as it has been in his career. He is obviously at the age where we will look at him individually as a player. We want him to be involved in our big games because of his experiences and his talent, and we'll always taper that recovery to allow him to be at the very best he can be."
That is absolutely true. Gerrard played 70 times in the Premier League over the past two years -- more than he had managed at any point since 2008. All that suggests, though, is that Rodgers is correct to assert that, in terms of fitness, Gerrard has looked after himself and been looked after well. What it does not do is prove the quality of his performances.
Looking at Gerrard's statistics for last season and what was probably his best campaign -- 2008-09, when Liverpool ran Manchester United to the wire in one of the great, forgotten title races -- is not particularly conclusive either.
His scoring rate dropped, but his pass completion increased, testament to a shift in his position and Rodgers' slightly less direct style. Perhaps the only compelling figure is that the number of passes he made in the final third, the sort of pass that marked Gerrard's style for so long, is significantly lower. Again, in part, that can be explained by the fact that he is playing deeper. It also hints, though, that his impact on games was reduced.
All that means is that the only way to gauge how well he is resisting the relentless beat of time is to use that most unreliable of indicators: the eye. Gerrard has become ever more susceptible to being harried out of possession; against Aston Villa and West Ham, the ease with which he was bypassed and beaten was marked. He looked like a man struggling to keep up. He has for a year or two.
Now, there is an important differentiation to make. To suggest that Gerrard is not the player he was or that he should no longer be an automatic selection for Liverpool is not to say that he should be dropped immediately or should never play again. Both of those conclusions would be ludicrously extreme.
A more sensible approach would be to learn the lessons that Milan did not. Rodgers' defiance when pressed on the subject suggests -- possibly incorrectly -- that he is not yet preparing for Gerrard's inevitable departure and that he has not yet accepted that his captain, like everyone else, will not be around forever. That needs to change. Liverpool must start to think about succession. They must start to think about what their world will look like without Gerrard. They must not fall into the comforting trap of believing in immortality. They must not make the same mistake as Milan.
The same could be said for the Premier League as a whole. A little more than a decade ago, when Milan Lab was at its peak, making players immune to age, Serie A as a whole became incrementally older. Teams had grown so used to success, so convinced by their greatness and so loyal to their stars that they refused to contemplate what would come next.
Milan's case was more spectacular than most, but it was a phenomenon that affected the entire league. On a weekend when Lampard scored the most pivotal goal in the Premier League, it is tempting to suggest that England is falling victim to the same thinking.
We do not want our stars to fade. We are frightened of what tomorrow holds. That fear must be confronted. The clock ticks. The sun sets.