How do the pigs master the art of walking on 2 legs

The pigs disappeared for a week, during which time, Squealer claimed to have been teaching them a new song....

It was a pig walking on his hind legs.
Yes, it was Squealer. A little awkwardly, as though not quite used to supporting his considerable bulk in that position, but with perfect balance, he was strolling across the yard. And a moment later, out from the door of the farmhouse came a long file of pigs, all walking on their hind legs. Some did it better than others, one or two were even a trifle unsteady and looked as though they would have liked the support of a stick, but every one of them made his way right round the yard successfully.