Nicodemus answered him, "How can these things be?"
From this word of Jesus, it is plain that people should be more concerned with obeying God's laws than in searching out the "why" and the "how." Nicodemus apparently permitted his puzzlement over the "how" of the new birth to prevent his ready acceptance of Christ's word.  Of course, such questions have their place, but obedience should not wait upon the resolution of all problems.