"He meant the rats that the Professor had driven crazy by forcing them to deal with problems which were beyond the scope of rats, the insoluble problems. He meant the rats had been trained to jump at the square card with the circle in the middle, and the card (because it was something it wasn't) would give way and let the rat into a place where the food was, but then one day it would be a trick played on the rat, and the card would be changed...

...but it is inevitable that they will keep changing the doors on you, he said, because that is what they are for; and the thing is to get used to it and not let it unsettle the mind. But that would mean not jumping, and you can't. Nobody can not jump. There will be no not-jumping. Among rats perhaps, but among people never."

— E.B. White, The Door