Jesus tells Nicodemus that unless a person is born anothen, he cannot see the kingdom of God. Nicodemus hears the saying in the sense of being born 'again' and asks how an adult can return to the womb.
John's Greek permits a second sense: birth 'from above.'
The adverb anothen can mean again, anew, or from above depending on context. John exploits such double meanings elsewhere in the Gospel. Jesus' further language of birth from water and Spirit and of the mysterious movement of the pneuma—a word that can mean spirit, wind, or breath—moves the conversation beyond physical birth.
The precise theological relationship of 'water and Spirit' has been interpreted in several ways; the text clearly locates the required birth in God's transforming work rather than a second physical birth.
The wordplay explains why Nicodemus' response is not simply foolish. He follows one legitimate meaning of Jesus' Greek expression while Jesus is speaking at a deeper level.
For the reader, both senses reinforce one another: entry into God's kingdom requires a genuinely new birth whose source is above, accomplished by the Spirit rather than by human ancestry or status.