Would you be brave enough to tell Bruce Willis he isn't so tough after all?
Bruce Willis made his film name as Die Hard cop John McClean
Well, Jonathan Hare is the man to let Hollywood's archetypal action hero know that in real life his stunts in blockbuster Die Hard would have left him... well, dead.
Dr Hare also has bad news for Keanu Reeves and Sandra Bullock, but at least some of Pierce Brosnan's stunts hold scientific water.
He travelled to Wales to deconstruct some of Hollywood's most outlandish stunts for hundreds of school pupils and a paying audience at a Cardiff cinema complex.
Dr Hare, whose invitation was part of National Science Week, pulled off the trick of using film clips to make science fun, while slipping a lesson in physics, chemistry and biology in at the same time.
He has made two BBC series for the Open University on the theme, called Hollywood Science, and hopes to do a third with co-presenter, Welsh actor Robert Llewellyn.
So what's the problem with Die Hard?
The 1988 all-action picture features Bruce Willis in every kind of death-defying derring do. Dr Hare examines one scene, where Willis' character straps a fire hose around his body and leaps off a tower block to escape terrorists.
Keanu Reeves' flying bus in Speed had quite a gap to fill
Dr Hare has worked out that Willis' body would have had to sustain about 10,000 kg, or about 100 times his body weight. Or enough to break his back, neck, arms, legs and ribs.
"Basically, don't do this at home. It's not what you want to do even when fighting international terrorists," said Dr Hare.
Another famous stunt, in the film Speed, saw Keanu Reeves and Sandra Bullock making a 50ft leap in a packed bus across a gap in the road.
If they had had a ramp at the ideal angle of 45 degrees, they would have made it. But the angle was nothing like that, and in reality the bus would have made an unscheduled and very sudden stop in a ball of flames 100 ft below.
But another striking scene in the Pierce Brosnan hit Dante's Peak got the scientific seal of approval.
That film shows Brosnan rowing across a lake which has been turned to acid by an erupting volcano. The acid is so corrosive that it burns through the aluminium boat.
According to Dr Hare, this is sound science. He cites one lake in Indonesia - Tiwo Nua Murii Koohi Fah - with a ph value of 0.3, "which is like battery acid".
A death-defying Bruce Willis looms on the screen behind Dr Hare
He said: "Acid lakes do exist and scientists go around the world measuring them. But although they look very beautiful they are not nice places."
In fact, the power of acid in food such as marmalade and tomato sauces is why you should not cook them in aluminium saucepans, because it removes the oxide layer.
The lessons of the event - sponsored by the Institute of Physics in Wales and the British Association for the Advancement of Science - seemed to have got through to pupils from five south Wales schools in the UCI cinema in Cardiff Bay.
"I think in James Bond films some of what goes on is extravagant," said William Docking, 14, of Bishop of Llandaff High School, Cardiff. "I think maybe next time I'll be wondering whether that's possible."
Fellow pupil James Jones, 15, said: "It'll give you a different aspect when you see a film."
Dr Hare, of the University of Sussex's Creative Science Centre, said it was most difficult to keep the interest of pupils in science in the GCSE years. Younger children did more experiments, and A-level students had already decided they were interested by choosing the subject.
But Hollywood need not worry that he is out to undermine its secrets.
"From my point of view, I'm not out to slag films off," he smiles. "It's just curiosity."