So what is a solid? Solids are usually hard because their molecules have been packed together. The closer your molecules are, the harder you are. Solids also can hold their own shape. A rock will always look like a rock unless something happens to it. The same goes for a diamond. Even when you grind up a solid into a powder, you will see tiny little pieces of that solid under a microscope. Liquids will move and fill up any container. Solids keep their shape.
In the same way that a solid holds its shape, the atoms inside of a solid are not allowed to move around too much. This is one of the physical characteristics of solids. Atoms and molecules in liquids and gases are bouncing and floating around, free to move where they want. The molecules in a solid are stuck in place. The atoms still spin and the electrons will still fly around, but the entire atom will not change position.

License information: CC BY-SA 3.0
MPAA: G
Go to source: https://en.wikibooks.org/wiki/Wikijunior:The_Elements/Solids