Get a block of butter, not spread. Put it in the fridge.

Take it out of the fridge and run a knife over it. It is quite a hard solid and it is difficult to scrape.

Now leave it for half an hour in a warm room or heat it for about 5 seconds in a microwave. Try the knife test again. It scrapes more easily now, so it has softened. But it changes shape only while you are moving it with the knife. So it is still a solid.

Butter is a mixture of substances. Some, but not many, melt at room temperature. Because only a few of the substances melt, the rest holds together as a solid.

The butter only becomes a liquid when it is heated and all of the substances in it melt.