How do the animals deceive Mr. Whymper in Chapter Seven in Animal Farm?
The animals use a number of tricks to deceive Whymper. They also lead him through the barn "on some suitable pretext," so he sees that the grain bins are full and feels that the animals have nothing to hide from him.
This plan to deceive Whymper is the work of Napoleon and reveals much about his character: he is more concerned, for example, with having people believe that the farm is doing well than actually working hard to make life on the farm better. In reality, Napoleon cares only for his reputation, as is made clear in the text:
Napoleon was well aware of the bad results that might follow if the real facts of the food situation were known.
In addition, Napoleon uses the sheep to spread his message to Whymper because he knows that they are easily manipulated. This is not the only time that he has used the sheep in such a manner: in Chapter Five, for instance, he had them bleat "four legs good, two legs bad" to silence the other animals during a discussion on the windmill.
Napoleon is, therefore, prepared to do whatever it takes to make Whymper believe his version of the truth.
By this time, the animals' lives are getting to be really hard. They don't have much to eat and they are having to work really hard. But they do not want the people around them to know this. So they decide to fool the people by fooling Whymper. They are counting on Whymper to spread the word that the animals are doing great.
So they do things like filling up the feed bins with sand and then putting a bit of food on top to make it look like the bins are completely full. They have various animals talk about how the rations have been increased at times and places where Whymper can hear.
That way, they make him think the farm is doing fine.