how does napoleon plan to keep the food shortage a secret to outsiders?

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

use 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.