Just how does Prince Philip keep going at 97, with no walking stick, no frame and only a heart stent to show for his age?
Is it good genes? A balanced diet? No waiting time to see a GP? A secret royal elixir? An elaborate system of levers and pulleys? Luck?
For much of his life, the Duke of Edinburgh has relied on a daily exercise regimen he learnt in the Royal Navy.
Known as the 5BX (Five Basic Exercises), the plan was developed in the Fifties by fitness pioneer Bill Orban for the Royal Canadian Air Force as a full-body strength and flexibility workout that requires no equipment, no warm-up, and no more space than the length of a bed.
It involves performing five exercises within 11 minutes, and increasing not the time but the repetitions as you get fitter.
First comes stretching for a minute: stand with feet hip-width apart and arms in the air, then bend forward and touch the floor, before stretching up and backwards.
Next, a minute of sit-ups, before you start back extensions: lie on your front with your hands under your thighs and palms upwards; raise one leg a few inches off the ground, then alternate.
Then it's press-ups for another minute and, finally, six minutes of running on the spot.
After every 75 steps, do 10 scissor jumps (stand with your right arm and left leg extended forwards and your left arm and right leg backwards. Jump up and change the position of the arms and legs before landing).
For beginners, anything between 100 and 175 steps is good.
After a month, though, you should be up to 400.
As training regimes go, it's old-fashioned but it has worked so well that both Prince Charles and Prince William have taken up 5BX.
Prince Philip completes his in addition to regular walking and carriage driving, a hobby he took up in 1971 after retiring from polo with an arthritic wrist.
Look through old photos and you'll notice Prince Philip's weight has almost never fluctuated.
His tailor of half a century, the small Piccadilly firm Kent, Haste & Lachter, has even confirmed the Duke's waist as 34in (86cm).
Another said he's still able to fit into the same naval uniform he wore on his wedding day.
The Duke's diet has been subject to fierce speculation, but observers have noted a resistance to carbohydrates, a fondness for fry-up breakfasts or oatcakes with honey, and no sweet tooth.
That said, he has an appetite: Darren McGrady, former chef to the Queen, tells a story of the Duke asking to swap his meal with the staff's, in order to get a bigger portion.
It is thought that every lit cigarette takes 11 minutes from a smoker's life expectancy, so Philip's decision - reportedly made almost overnight - to quit smoking prior to his marriage to the Queen in 1947 ought to have done him the power of good.
So, too, is his reputation for only drinking alcohol in moderation, a restraint that's not always been shared by his relatives.