* Solid objects must have a definite shape. Think of a rock, a book, or a chair. They maintain their shape because the particles within them are tightly packed and don't move freely.
* Liquids take the shape of their container, so they don't have a definite shape on their own.
* Gases also take the shape of their container and expand to fill the available space.
So, the answer really depends on what category of matter we're discussing.
