The King of Elfland's Daughter by Lord Dunsany

In those days Ziroonderel would amuse the boy by charms and by little wonders, and he was content for a while. And then he began to guess for himself, all in silence, where his mother was. He listened to all things said, and thought long about them. And days passed thus and he only knew she had gone, and still he said never a word of the thing with which his thoughts were busy. And then he came to know from things said or unsaid, or from looks or glances or wagging of heads, that there was a wonder about his mother's going. But what the wonder was he could not find, for all the marvels that crossed his mind when he guessed. And at last one day he asked Ziroonderel.

And stored though her old mind was with ages and ages of wisdom, and though she had feared this question, yet she did not know it had dwelt in his mind for days, and could find no better answer out of her wisdom than that his mother had gone to the woods. When the boy heard this he determined to go to the woods to find her.