Rocks, Photons, and Spacetime Motion
Newton's first law is often taught through straight-line motion, but the deeper point is about what counts as a force and what path the body follows when no force changes its state.
A thrown rock is not an exception to inertia. Gravity changes the rock's trajectory, so the path curves relative to the ground. In a spacetime description, the same event can be read through a path rather than only through a flat, straight-line picture.
The useful teaching point is that motion models depend on the chosen description. A photon, a falling rock, and an object moving without contact forces are not the same physical case, but each forces the model to say what is being held constant and what interaction is changing the path.
That is why the example is valuable: it pushes the explanation from memorized wording toward the structure of the model.