Understanding singularities involves new techniques. One has to understand how a generic spacetime evolves. To do so we review important point: convergence of geodesics and the Raychaudhuri equation. We calculate some example of these equations in appendix 
to aid ones physical understanding.
Our interest in curvature comes from the fact that the light rays are obliged to converge because of it. They are bunched together.
Therefore, there can exist spacetimes with region where even light cannot escape. An example is black hole spacetimes(see chapter page ).