* Solid: Molecules in a solid are tightly packed together and have a fixed, rigid structure. They vibrate in place but have limited movement, resulting in slower average molecular speed.

* Liquid: Molecules in a liquid are less tightly packed than in a solid. They have more freedom to move around, slide past each other, and collide. This increased movement leads to a higher average molecular speed.

Think of it this way:

* Imagine a packed dance floor (solid) - everyone is moving slightly, but mostly just swaying in place.

* Now imagine a crowded but open space (liquid) - people can move around, bump into each other, and change positions more freely.

This difference in molecular movement is why liquids flow and solids don't.