Conversation between three men of the family:
Son #2: So black holes are real?
Dad: Sure they’re real.
S2: So what are they?
Dad: They’re places where the gravity is so strong that the light doesn’t have enough energy to get out; so they’re black.
S2: Then what happens to the light?
Dad: It just disappears.
Son #1 interjecting from the hallway: That’s not right, the black hole gains mass from the light.
Dad: (surprised) That’s right, the photons add to the mass of the black hole.
Son #2: So as the black hole gets bigger, the gravity gets stronger.
Dad: Right (backing slowly towards the door).