I remember when I was a kid reading about how people used to claim they had horses that could do math. They would say "what is 2+1 and the horse would stamp its foot 3 times. If you watched what was happening closely you could see the person doing something like smiling or nodding when the horse got to the right number. The horse picked up on the nearly imperceivable reaction to know when to stop.
Very relevant to this thread.