In Animal Farm by George Orwell, what lies do Napoleon and his pigs tell the other farm animals in order to excuse their acts of trade with humans?...

The notion of trade with humans is introduced in Chapter Six. In actuality, most of what the pigs told the other animals about the necessity to trade with humans was based on realistic facts. The farm was indeed running short on many essentials which could not be produced on the farm, as seen in the following extract:

Nevertheless, as the summer wore on, various unforeseen shortages began to make themselves felt. There was need of paraffin oil, nails, string, dog biscuits, and iron for the horses' shoes, none of which could be produced on the farm. Later there would also be need for seeds and artificial manures, besides various tools and, finally, the machinery for the windmill. How these were to be procured, no one was able to imagine.

It was for this purpose that Napoleon announced one Sunday morning that they would engage in trade with the neighbouring farms. This would not be done for commercial gain, but to ensure the availability of whatever essentials the farm required. He stated that the needs of the windmill should be a priority and it was thus a necessity to conduct trade with the humans.

The announcement was, however, met with some resistance since the animals felt vaguely uneasy that not to trade had been one of the primary warnings issued by Old Major in his speech. Four pigs wanted to protest but were quickly silenced by intimidating growls from Napoleon's dogs and the sheep's incessant bleating.

Squealer, however, found it necessary to go around to set the animals' minds at ease and in this instance, he told a blatant lie. He knew that the animals lacked the memory to recall exactly what it was about dealing with humans that was rejected and challenged their recollection thereof, as stated in this extract:

He assured them that the resolution against engaging in trade and using money had never been passed, or even suggested. It was pure imagination, probably traceable in the beginning to lies circulated by Snowball. A few animals still felt faintly doubtful, but Squealer asked them shrewdly, "Are you certain that this is not something that you have dreamed, comrades? Have you any record of such a resolution? Is it written down anywhere?" And since it was certainly true that nothing of the kind existed in writing, the animals were satisfied that they had been mistaken.

This was a common tactic by the pigs and it had become Squealer's trademark. The animals were forced to accept whatever was told to them since there was no evidence to support the contrary. No commandment existed in this regard and, as it was, it would have been altered in some way to suit the pigs' agenda anyway. In the end, Napoleon employed a solicitor from Willingdon, a Mr. Whymper, to act as a middleman between Animal Farm and the humans.

It is apparent that the pigs knew precisely how to manipulate the animals. They targeted their weaknesses, of which a lack of intelligence was one and their poor memories another. Since the animals were at a loss in both regards, they did not stand a chance in arguing against what they were told. 

Another method the pigs used was to threaten the animals by using Napoleon's dogs. The animals had already witnessed Snowball's brutal expulsion earlier and were afraid that the same could happen to them. Furthermore, the sheep had become convenient tools to silence opposition by their continuous bleating of 'Four legs good, two legs bad.' 

This kind of manipulation became the order of the day and the general animal public became mere puppets in the pigs' manipulative trotters. These tactics ensured that they retained power and live lives of privilege and luxury whilst the other animals slaved away, cold and hungry. They were no better off than they had been in Jones' time.