Five Children and It by Edith Nesbit

Cyril had once pointed out that ordinary life is full of occasions on which a wish would be most useful. And this thought filled his mind when he happened to wake early on the morning after the morning after Robert had wished to be bigger than the baker's boy, and had been it. The day that lay between these two days had been occupied entirely by getting the governess-cart home from Benenhurst.

Cyril dressed hastily; he did not take a bath, because tin baths are so noisy, and he had no wish to rouse Robert, and he slipped off alone, as Anthea had once done, and ran through the dewy morning to the sandpit. He dug up the Psammead very carefully and kindly, and began the conversation by asking it whether it still felt any ill effects from the contact with the tears of Robert the day before yesterday. The Psammead was in a good temper. It replied politely.