‘Nicodemus says to him, “How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?” '
Nicodemus takes what Jesus says to mean born again physically, and speaks as though he confuses this with natural birth. John often uses a question arising from a misunderstanding to illuminate a truth. So Nicodemus asks, ‘How can an old man enter his mother's womb a second time?' He is probably simply seeking more information. He does not understand what Jesus means, and deliberately makes it sound enigmatic.