Material World - 24/12/2009

Can crunching celery undo the damage of too many mince pies? How fast is gravity? Why is slug slime sticky? And how much matter can a black hole suck in before it explodes? Quentin Cooper is joined by palaeontologist Mike Benton, atmospheric physicist Helen Brindley and cosmologist Gerry Gilmore for a special question and answer edition of the programme.