 A careful definition of a relativistic black hole will therefore rely only on the geometrical features of spacetime. We will need to be a little more precise about what it means to be “a region from which nothing, not even light, can escape”. First, there will have to be someplace to escape to if our definition is to make sense. The most common method of making this idea precise and rigorous employs the notion of escaping to infinity. The idea is that if a particle or light ray cannot travel arbitrarily far from a definite, bounded region in the interior of spacetime but must remain always in the region, then that region is one of no escape, and is thus a black hole. The boundary of the region is the event horizon. Once a physical entity crosses the event horizon into the black hole, it never crosses it again. 