There are battle tanks, septic tanks, think-tanks but none are as fascinating as the invisible tanks embedded inside performers. The ones with fuel which never seems to run out. One night at the Kennedy Centre in 2009, the comedian Jon Stewart spoke elegantly about this when he honoured the enduring singer Bruce Springsteen.
"Whenever I see Bruce Springsteen do anything," he said, "he empties the tank every time... He empties that tank for his family, he empties that tank for his art, he empties that tank for his audience, and he empties it for his country."