So far, we have made intensive use of the existence of local inertial systems in the definitions and derivations of tensors and derivatives, without explicitly addressing the aspect of curvature, which we also need to introduce precisely. Since we can only perceive curvature with our limited sensory organs when it manifests in one-dimensional (e.g., curves) or two-dimensional (e.g., surfaces) objects (we cannot imagine a three-dimensional space as curved, let alone four-dimensional spacetime), we want to explain some of the terminologies and concepts using simple examples before we then define them exactly in four-dimensional spacetime.