At two o'clock a bank robber stole in.  "This is a holdup," the man said rudely.  He took a gun from under his jacket, pointing to George.  "Hand it over!" George reached into his money-box and took all the bills from the top part-close to six thousand dollars.  The robber snatched them and turned to leave. Then while everyone watched the robber, George calmly lifted the top part of the money-box, took hills from the bottom part and put them into his own pocket secretly. The door was shut and the bank robber was gone.  George fainted. As soon as he was safely behind his bedroom door, George counted the money.  He had eight thousand dollars.  He was very happy. The next morning, while the others were examining the bank's records, George was called into Mr.  Burrow's office and was introduced to Mr.  Carruthers, who used to be president of the bank. "Good morning, George, I was sorry to give you a hard time yesterday, but with all the banks being robbed these days I thought it would be a good idea to prove that our little bank can be robbed, too.  I retired yesterday, just to keep everyone on his toes.  Now, I have put the money back in your money-box-all six thousand. " In this article "to keep everyone on his toes" means "  _  ".
to make everyone work hard to keep everyone standing straight to make everyone do a kind of exercise to keep everyone paying attention to the coming danger