文档介绍：
A little Scotch boy was sitting in his grandmother’s kitchen. He was watching the red flames in the wide open fireplace and quietly worndering about the causes of things. Indeed, he was always wondering and always wanting to know.
”Grandma,” he presently asked, “what makes the fire burn?”
This was not the first time he had puzzled his grandmother with questions that she could not answer. So she went on with her preparations for supper and paid no heed1 to his query2.
Above the fire an old-fashioned teakettle was hanging. The water within it was beginning to bubble3. A thin cloud of steam was rising from the spout4. Soon the lid5 began to rattle6 and shake. The hot vapor puffed7 out at a furious rate. Yet when the lad peeped under the lid he could see nothing.
”Grandma, what is in the teakettle?” he asked.
”Water, my child -- nothing but water.”
”But I know there is something else. There is something in there that lifts the lid and makes it rattle.”
The grandmother laughed. “Oh, that is only steam,” she said. “You can see it coming out of the spout and puffing up under the lid.”
”But you said there was nothing but water in the kettle. How did the steam get under the lid?”
”Why, my dear, it comes out of the hot water. The hot water makes it.” The grandmother was beginning to feel puzzled.
The lad lifted the lid and peeped inside again.