This is a bit of a tricky question! It depends on how you're thinking about it.

Here's why:

* The traditional opposite of solid is liquid. This is based on the three states of matter: solid, liquid, and gas. Liquids flow and take the shape of their container, while solids hold a definite shape.

* But gas is also the opposite of solid in a different way. Solids have a fixed volume and shape, while gases have neither. They expand to fill their container.

So, depending on what aspect of "opposite" you're considering, either liquid or gas could be the answer.