Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
What is a black hole, and what would happen if you fell in? (for a curious adult who has not studied physics). Cover, in this order: What is a black hole?; How can gravity be strong enough to trap light?; How does a black hole form?; What is the event horizon?; What happens if you fall into a black hole?; If nothing escapes, how do we see black holes?; Do black holes really suck everything in?; Can a black hole die? — page 6: Seeing what cannot be seen
Nothing escapes from inside, so the hole is dark. But like wind, you can't see it directly. You watch what it does to everything around it.
Clue one: glowing gas. Falling material spins into a disk, rubs against itself and heats to millions of degrees, so it glows just outside the horizon where we can see it.
Second clue: stars near the galaxy's center whip around a hidden point so fast that millions of Suns' worth of mass must be packed into a tiny space.
Third clue: bent light. A black hole's gravity bends the starlight passing near it, so a distant star's image gets warped and stretched.
Fourth clue: ripples. Two black holes merging shake spacetime itself. Detectors feel that tiny stretch, and it reveals the collision with no light needed.
So here is the whole picture: one dark center, and four ways it gives itself away, glowing gas, fast orbits, bent light, and ripples.
So a black hole is invisible only to the eye. Its gas glows, its gravity steers stars and bends light, and its collisions shake space, so it can be found by what it does.