When the World Expo opens in Zaragoza, Spain, next year, fairgoers will encounter a building with walls made from water.

The water will come from thousands of little jets that can be switched on and off, rapid-fire, by computer-controlled sensors.

The resulting effect will enable images and text to scroll in the water walls. Or as a person approaches, the sensors could shape the water flow to make a door appear anywhere in the wall, and then close it after the person ambles through.