And so it was, that, while they were there, the days were accomplished that she should be delivered. This little town housed the true bread of life for all mankind. Jesus was laid in a manger when He was born. It is that from which animals ate in the stalls. In other words, the baby laying in an actual manger would be sign.
Signs teach a lesson.
The term SIGN used by the angel literally means a token. A symbol of something spiritually understood. It is a picture of Jesus as the food to be eaten. A manger is where food is put for feeding. He is the bread of life.