How Do We Know What's Inside a Black Hole?

Imagine a giant drain in the ocean that sucks in everything. You cannot see what is at the bottom because of a thick fog (the event horizon), but you can tell how big it is by watching how fast things swirl around it.

Scientists use three main tricks to figure out what is inside. First, they watch stars orbiting nearby. If a star moves very fast in a tight circle, the black hole must be heavy. Second, they listen for ripples in space called gravitational waves. When two black holes crash together, they make a sound that tells us their size and how fast they spin.

There is a rule called the No Hair Theorem. It says a black hole only has three features: mass, spin, and charge. That is like saying a cat is just defined by being fat, running fast, or having static electricity. Everything else (like color or shape) gets swallowed up. So even though billions of tons of matter fall in, they all lose their unique identity.