Years ago I was on the tube, the Piccadilly Line from Heathrow into downtown London.
Knowing the trains are packed with tourists, the buskers love that line. Sure enough, about three stations out of Heathrow, a guy got on the train, announced that he'd be collecting any donations after his song, and started playing the violin.
Afterwards, he went around with a cap. An American tourist with me gave him 50p and then, after he went away, asked me if she'd given him enough.
I said yes, and that she didn't have to give anything if she didn't want to.
After all, I added, you can always tell the regular commuters, plugged into their iPods, books or newspapers open, conspicuously ignoring the little hat being waved in front of them.
She was relieved that she hadn't committed a tipping faux pas.
"It's just so different in New York," she said. "If we see a guy get on the train with a violin case, we duck!"