question:  Why does Grannie fool Billy as well when she rides away with Park?
context: DOUBLE TROUBLE by CARL JACOBI Grannie Annie, that waspish science-fiction writer, was in a jam again. What with red-spot fever, talking cockatoos and flagpole trees, I was running in circles—especially since Grannie became twins every now and then. Grannie Annie strode in the lead, her absurd long-skirted black dress looking as out of place in this desert as the trees. He could be excused this time, however, for this was only our third day on Jupiter's Eighth Moon, and the country was still strange to us. But Miss Flowers, known to her friends as Grannie Annie, had always been mildly crazy. Blood-and-thunder as these books are, however, they have one redeeming feature—authenticity of background. Grannie Annie was the original research digger-upper, and when she laid the setting of a yarn on a star of the sixth magnitude, only a transportation-velocity of less than light could prevent her from visiting her "stage" in person. He was an old prospector Grannie had met, and he had become so attached to the authoress he now followed her wherever she went. As for Xartal, he was a Martian and was slated to do the illustrations for Grannie's new book. And then all at once Grannie stopped again, this time at the top of a high ridge. "Billy-boy," she said to me in a strange voice, "look down there and tell me what you see." Attached to its hood was a taut wire which slanted up into the sky to connect with the kite. Five minutes later Grannie was shaking his hand vigorously and mumbling introductions. Occasionally he touched a stud on an electric windlass to which the kite wire was attached. Apparently these adjustments moved planes or fins on the kite and accounted for the car's ability to move in any direction. "They walk out into the Baldric," he continued, "and nothing can stop them. Fully two hundred cockatoos were perched upon it.
output:  She didn't. It was one of the cockatoo images.
options:  She didn't want Billy to know where they were heading.  She had to pretend she was replaced by a cockatoo, and make it convincing.  She didn't want Antler to know about the cockatoo images and how they worked.  She didn't. It was one of the cockatoo images.