It's a question that tempts an answer such as: "If I knew that love, I'd be speaking to you from the beach in Mustique."
But here's the thing. I do know - and I'm prepared to tell. It's big and it's bad for advertising as we know it.
According to estimates, half the population did some of their Christmas shopping online last year.
Reservations about dicey credit card security or getting stuck with shoddy goods that don't fit are evaporating from the population.
Online shopping will continue to usurp the high street as doubters diminish and online service improves.
Now, where in-store "service" involves some gormless kid drifting over to sing "You alright there?", this is no loss.
But a good sales assistant can really help. And for the retailer, a well-timed intervention as the shopper is about to give up is an important business asset.
Yet online, customers are not served but enabled. It's the ultimate self-service. So if they get to the checkout and have second thoughts, the online retailer can only sigh as he peruses his abandonment metrics.
A small web company called Talisma has a cool solution. As the shopper noodles around the site, Talisma's software opens a speech bubble on the page offering help - either an e-mail exchange or a call.
On the other end of the message is a contact centre manned by well-trained operatives, armed with invaluable, possibly personalised, service information.
So here's the really big thing, one step on from here - direct interaction between the brand owner or dealer and the online shopper.
Of course, the customer can turn them off, but for the serious buyer, it is pearls from the source.
As a serious seller, what better way of marketing? Forget the focus groups to establish average customers' purchase hang-ups - have it out one to one.
Switch the call centre from responsive (eventually) to proactive. Switch advertising money from speculative display or direct marketing to personalised engagement with individuals self-defined as customers.
We pundits can get enchanted by an idea a week, and just move on.
But I've put my money where my column is and co-founded the company that will make this happen. Mustique beckons.
Richard Eyre is a media pluralist, firstname.lastname@example.org