Here's why:

* Definite shape and volume: Trees maintain a distinct shape and occupy a fixed volume. They don't readily change their form like liquids or gases.

* Rigidity: The wood of a tree is strong and resists deformation. It can support its own weight and withstand external forces.

* Molecular arrangement: The molecules in wood are closely packed and arranged in a rigid structure, which gives it its solidity.

While a tree might appear somewhat flexible, especially in the wind, its overall structure remains solid and retains its form.