Here's why:

* Solids have a definite shape and volume. They don't change their shape easily. Nuts and bolts maintain their shape, even when you try to bend or deform them.

* Liquids have a definite volume but take the shape of their container.

* Gases have no definite shape or volume, expanding to fill their container.

Since nuts and bolts hold their shape and don't flow like liquids or spread out like gases, they are classified as solids.