Matter does not always stay the same. It can change from one state to another. A solid can become a liquid, and a liquid can become a gas. It is easy to see these changes with water.
    Ice is solid water. If you heat a piece of ice, it melts and turns into a liquid. 
    If you heat the water more, it boils and turns into steam.
    物体不只有一种状态，而能从一种状态变化为另一种状态。固体会变成液体，液体能变成气体。我们以水为例子可以更容易看出来。
    冰是固体的水，如果你加热一块冰，它会融化并成液体。
    如果你继续给水加热，它会沸腾并转化成蒸汽。