Explain one of the tricks used to convince Whymper and the outside world that conditions at the farm were good?

'It was vitally necessary to conceal this fact from the outside world. Emboldened by the collapse of the windmill, the human beings were inventing fresh lies about Animal Farm. Once again it was being put about that all the animals were dying of famine and disease, and that they were continually fighting among themselves and had resorted to cannibalism and infanticide. Napoleon was well aware of the bad results that might follow if the real facts of the food situation were known, and he decided to make u se of Mr. Whymper to spread a contrary impression. Hitherto the animals had had little or no contact with Whymper on his weekly visits: now, however, a few selected animals, mostly sheep, were instructed to remark casually in his hearing that rations had been increased. In addition, Napoleon ordered the almost empty bins in the store-shed to be filled nearly to the brim with sand, which was then covered up with what remained of the grain and meal. On some suitable pretext Whymper was led through the store-shed and allowed to catch a glimpse of the bins. He was deceived, and continued to report to the outside world that there was no food shortage on Animal Farm.'