The plain on the northern continent of the world was covered in them - a grid of stone cubes, each cube a kilometre across. They were lit from within by unknown energies, bright enough to be seen from orbit. At first the crew of the Niven had thought the cubes were arcologies - self-contained cities for the inhabitants of the planet. But when the Niven had signalled the world, there had been no response. Lasers, radio, even pulsing the ship's fusion thrusters. All had been ignored. Finally, Captain Mistry made her decision.

"We land," she said, "and we investigate."

The Niven landed at the edge of the grid, on an open part of the plain. If the crew had hoped that this would elicit a response, they were disappointed. In the captain's words: "Here we are - visitors from another world. If this had been Earth, we would be surrounded by the curious. Here? Nothing?"

They embarked on the next stage of the protocol. Volunteers were asked for; brave souls willing to enter the grid of stone cubes and to survey them. A team of a dozen left the Niven and headed for the array.

If the grid of cubes had been an impressive sight from orbit, from the ground they were overwhelming. Up close, each cube rose into the alien sky, dwarfing the humans as they walked among them. They were geometrically perfect. Their surfaces were smooth, without blemishes or openings. If there was anyone - or anything - within, it was sealed away for ever.

On the fourth day of the expedition there was a breakthrough. While trying to obtain a sample from a cube for analysis, one of the technicians touched its side. Instantly the cube went dark - as did those surrounding it! In the ensuing darkness, the survey crew panicked.

"What did you do?" the leader demanded.

"I touched it," the hapless crewman replied. "Like this." And, as he demonstrated, the cube and those surrounding it began to glow again.

Captain Mistry gave her permission for the survey crew to investigate further. By a series of experiments, they determined a set of rules for the cubes.

1) They only responded to the touch of a sentient being. Drone and probes had no effect.

2) The cube that was touched changed state. If it was illuminated, it went dark. If it was dark, it lit up.

3) When the cube that was touched changed state, those immediately surrounding it did the same, going from light to dark or dark to light.

Could it be that the cubes were some kind of intelligence test, designed for visitors to the world? There was only one way to find out. The ship's computer was set to determining a sequence that would change all of the cubes in the array so that they were dark, and the instructions were passed to the survey crew. As the last of the cyclopean structures went dark, the airwaves were filled with a cacophony of electronic noise, then silence. The array flashed three times, the brilliance reflected from the clouds above like summer lightning, and then ...

The cubes lit up. This time, the array had formed a checkerboard pattern of alternating light and dark.

Captain Mistry groaned in frustration.