What do pebbles thrown at the White Rabbit's house become?

Pebbles thrown at the White Rabbit’s house become small cakes.

Alice has grown enormously after drinking from a bottle inside the Rabbit’s house, becoming trapped indoors. The White Rabbit and his neighbours try several ways to remove her, including sending Bill the Lizard down the chimney.

When those efforts fail, the crowd throws pebbles through the house. Wonderland’s logic transforms the stones into small cakes when they land. Alice eats one, shrinks back to a manageable size, and escapes into the forest.

The scene is easy to confuse with the other edible objects in the book: the “Eat Me” cake makes Alice grow, while the mushroom later lets her change size in either direction. The pebble-cakes are specifically a way of driving Alice out of the Rabbit’s house.