What do Hogwarts and Tottenham Court Road tube station have in common? Moving pictures, that's what.
On a recent trip to London I found that all of the advertising posters on the Northern Line escalator at Tottenham Court Road tube had been replaced by LCD displays.
We've become accustomed to seeing these as one-off advertising displays in post offices and shops.
However, to be suddenly confronted by 40 or 50 of them doing no more than cycling through adverts for West End Shows, recruitment companies and sunglasses made me realise just how ubiquitous this new media is about to become.
And it is a new media. The glorified Power-point presentations that currently run on these screens are just the start. Advertisers are currently just lifting one media, the poster, and dropping it into a new format.
They aren't yet looking at what this new media is capable of, what it can do differently, and then exploiting that.
We also need to be clear that these displays aren't simply video screens. Customers are in view of each one for a matter of seconds and the screens are silent. Everything must be conveyed visually, and in seconds.
Which brings us back to Hogwarts. There the pictures move, the Fat Lady smoothes the folds of her pink satin dress, but the pictures aren't videos.
The movement is enough to catch the eye, to bring the scene to life. But the scene itself doesn't change. This is probably the approach that these new screens require.
A picture of a tropical beach where the waves crash slowly against the shore, a woman slowly drinking a new alcopop. Subtle movement. Moving pictures.
This need to develop an aesthetic of moving pictures is not something that will be limited to advertising displays.
We already have it to a degree with screensavers - the aquarium, the man on a desert island, simple images constantly moving. Soon we'll have it in the home as well.
At the recent CEDIA Smart Home Exhibition at the NEC one of the star attractions was an A2 sized picture frame - pine border, white card mount, but the centrepiece wasn't a watercolour but an LCD screen, playing a constant loop of still images and video clips from a RAM chip.
You might be happy with a rolling slide-show of still images, you might even want short clips of your favourite films or home-movies. But what is likely to work best is something in between - a moving picture.
And once we've mastered the art of moving pictures we then have to decide how smart we want them to be.
Many years ago a US lab developed a "smart" notice board. Employees wore badges which the noticeboard detected, and then showed notices on its video screen relevant to that employee. We could do that cost-effectively now.
It just needs on of these screens and an RFID chip. A smart screen could even detect your phone or the RFID chips on things you've bought; the possibilities, and infringement of civil liberties, are no doubt immense.
The screens may not even need your co-operation.
A simple webcam hooked up to some image processing software might be able to work out your age, sex, dress style and even carrier bag logos, and start playing ads targeted at you.
So when you next pass that old static bill-board take a moment to savour its passivity. Next time it might turn and look back, and tell people around you more about yourself than you'd like them to know.