"And she shall bring forth a son, and thou shalt call his name Jesus" (Matthew 1:21). At this time of year many ask, "How did God become a man? What happened to make it possible for Jesus to be born of a woman?"
The Bible doesn't really give much information about how the incarnation of Jesus took place. We are told that the Holy Spirit came upon Mary and that the power of the Highest overshadowed her.
"And the angel answered and said to her [Mary], 'The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God'" (Luke 1:35).
That's really all we know about how Jesus became a man, and it might not be a sufficient answer to those who raise this question. However, if you're confronted with such questions, don't avoid them. Give the true answer: We don't fully understand how God became a man. Then follow that statement by reminding the questioner that what we do know is that God, who created the earth and all that is in it, is capable of such a miraculous act.