Here's why:

* Internal energy is the total energy of the molecules within a substance, including their kinetic energy (energy of motion) and potential energy (energy due to position and interactions between molecules).

* Expansion means the molecules are moving further apart. This requires work to be done, which is provided by the internal energy of the air.

* As the air expands, its molecules move faster, increasing their kinetic energy, and therefore increasing the internal energy of the air.

Note: This is generally true for ideal gases, where there are no intermolecular forces. In real gases, some internal energy might be lost due to intermolecular forces during expansion. However, the overall trend is that internal energy increases with expansion.