The Message clarifies it for us: John 3:3-6 (MSG)
3 Jesus said, "You're absolutely right. Take it from me: Unless a person is born[spirit] from above, it's not possible to see what I'm pointing to—to God's kingdom." 4 "How can anyone," said Nicodemus, "be born[spiritual] who has already been born and grown up? You can't re-enter your mother's womb and be born again. What are you saying with this 'born-from-above' talk?" 5 Jesus said, "You're not listening. Let me say it again. Unless a person submits to this original creation—the 'wind hovering over the water' creation, the invisible moving the visible, a baptism into a new life—it's not possible to enter God's kingdom. 6 When you look at a baby, it's just that: a body you can look at and touch. But the person who takes shape within is formed by something you can't see and touch—the Spirit—and becomes a living spirit.