Neither Powell nor Donovan had spoken to a subsidiary robot, because they were controlled almost wholly by Dave. They were almost as close to Dave as his fingers. In fact, Powell and Donovan often called them Dave's"fingers'. Donovan fetched one of the subsidiaries. Powell looked at DV-5-2. "Several times recently, your boss stopped working unexpectedly. Do you remember? "Yes, sir.' "What were you doing each time? The"finger" answered the question automatically, without interest. "The first time, we were working on a difficult rock in Tunnel 17. The second time, we were supporting a roof in Tunnel 12. The third time, we were preparing to tunnel at a new level. And the fourth time was after a fall of rocks in Tunnel 17." "What happened at these times?" "I can't really describe it. An order was given, but before we could receive it, we were ordered to march or dance. 'Why?' The subsidiary robot shook his head unhappily. don't know, Powell sat back. "All right. Get back to your boss. The"finger" left and Powell turned to Donovan. "We've got to catch that rabbit. We've got to get out there and watch them all the time. And wait for something to go wrong. "No," Donovan said suddenly. "We can make Dave go wrong, while we're watching.' Powell was puzzled. "How?" Think about it. You've got the brains, you think. When