Without moving your head, look back and forth between the two pencil tips.
The result was a duelling game called Little Bandits in which you controlled your character by moving your head.
"What I like about it is that it's accessible, you can control it just by moving your head; that's an interesting step.
Immersed in the sound, it's possible to play a crude melody simply by moving around the space or by sitting still and moving your head up and down or side to side.
With or without glasses, there's the brow-furrowing fact that to fully appreciate 3D involves moving your head from side to side slightly, to see objects in the foreground moving independently of those in the background.
Clooney once spoke with Steven Spielberg on the set of "E.R".; Spielberg watched his performance on a monitor, and, tapping the screen, said, "If you stop moving your head around, you'll be a movie star".
But with the Minecraft demo, it was a little frustrating, viewing this vast and beautiful 3D model, but having to keep moving your head to keep it in the centre of your vision so that the whole thing didn't disappear.
"Cataract operations are the only ones where handholders are essential, as moving your head even fractionally is not a good idea," Yvonne Marlow, who runs the volunteers' group, told me.
You operate it with voice or by moving your head around to navigate or select menu options.
In both the time trial and velodrome events, there is less need to worry about moving your head, says Matt Seaton, author of cycling memoir the Escape Artist.
