Everybody's excited about gravitational waves, but why? What do they mean? How were they found, with lasers? How can we possibly know they came from two black holes a billion years ago? How can a whole sun's worth of energy have been used up? How can gravity "wave", as it were? What do colliding black holes sound like? (Much more musical than you'd expect, it turns out.)Claire has some questions, and Chris has... Well, Chris has a chat with astrophysicist Katie Mack, of @AstroKatie fame, and she has the answers.