How can water change between ice and liquid?

An ice cube can become a puddle without becoming a different substance. Both ice and liquid water are made of water molecules. A molecule is a tiny unit of water, far too small to see in an ordinary glass.

In ice, the molecules form an organized structure. They vibrate around positions rather than sitting perfectly still. This structure helps ice hold its shape. In liquid water, molecules can move past one another, so the water flows and takes the shape of its container.

When energy transfers into ice from warmer surroundings, the structure can loosen and melt. A warm room can provide this energy without a flame. During melting, ice and liquid water can exist together; a whole cube need not change at once.

Freezing happens as energy transfers out to colder surroundings and the molecules form the solid structure again. The pictures keep the same number of water symbols in both states. They show a change in arrangement and movement. The molecules remain water through the journey from ice to liquid and back.