Warmer atoms expand. There is also more movement. This is also explained when putting it into terms with solid vs. gas. in a solid, the atoms vibrate, but they can't move very far because there is not a lot of space between them. In a gas, on the other hand, the atoms are spread apart, and they move very fast. Putting it into context with water, the steam (gas) version is hot, and the ice cube (solid) version is cold.
Warmer atoms expand. You can think of it like a warm summer day you don't want to be next to your friends because it's hot out. On a cold day you try to stay closer to your friends because you produce body heat and keep each other War
