Here's why:

* Liquids have a definite volume but not a definite shape. Soup takes the shape of its container but always has a certain amount of liquid.

* Liquids can flow. You can pour soup from one bowl to another.

* Liquids are compressible to a small extent. While you can't really compress soup significantly, liquids are slightly compressible compared to solids.

While soup contains solid ingredients like vegetables, meat, or noodles, the base of the soup is a liquid broth or stock.