How did the lamp post get in to Narnia?

In the book "The Magician's Nephew", two young London children are somehow drawn into a "wood between the worlds," which is a central place from which all other worlds can be accessed. The boy, Digory, disobeys an edict not to ring a bell when he and Polly enter Charn, and by ringing the bell, they awaken the White Witch (Jadis of Charn).

When returning to their own world, Jadis follows the children, finding herself in London, where she is a tall and powerful woman. In rage, she breaks off piece of a lamppost and when they return to Narnia she is still holding it in her hand.

When Aslan starts to create the world, she is so terrified of this "singing lion" that she throws the lamppost piece at Aslan and runs away. The lamppost has absolutely no effect on Aslan, but because the whole of Narnia is just bursting forth with new life, like the rest of the trees the lamppost begins to grow up from out of the ground, becoming a life of its own with a light that never dies.