Jesus told Nicodemus...
John 3:3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
[5] Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
The gospel of Mark says...
Mark 16:16 He that believeth and is baptized shall be saved.
Nicodemus asked, how can a man enter into his mothers womb and be born again?
He had no idea that Jesus was speaking of mans natural birth, by water through woman, and that the spiritual birth through Christ must added to that to be born-again.
Born-again is a spiritual birth, as Paul said that takes place in the heart. Jesus was showing Nicodemus that this birth must be added to the natural birth.