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 3: How black holes form
A star is a tug-of-war. Gravity pulls inward, fusion's heat pushes outward, and for millions of years the two stay balanced.
A massive star fuses lighter atoms into heavier ones, until its core becomes iron. Fusing iron takes energy instead of releasing it, so the outward push fades.
With the push gone, gravity wins. The core falls in on itself in a fraction of a second, squeezed to staggering density.
Shrink the same mass into less space and the escape speed climbs. Past light speed, nothing escapes, not even light. That is a black hole.
The collapse sets off a supernova, blasting the outer layers away. That explosion doesn't make the black hole; the crushed core left behind does.
So a black hole is what is left when a giant star loses its fight with its own gravity: nothing new is added, just a core squeezed until light itself cannot leave.