A 根据下面短文回答下列问题。 A bird was very thirsty (渴). He was looking for water. He saw a bottle and flew down happily. He saw some water in the bottle. But he could not get the water. "What can I do" thought the bird. He looked around and saw some pebbles (鹅卵石). Then he had a good idea. He dropped the pebbles into the bottle one by one. Now he could drink the water happily. The bird put some ______ into the bottle.