How do black holes work? And how do we learn about them, if they're impossible to see? We talk to Dr. Karl Gebhardt, who has helped discover over half of the black holes that astronomers have ever found. Light cannot escape from a black hole, which means that they are impossible to see. We find out the secret to finding black holes, and how what happens inside might explain mysteries of gravity. Astronomy adventures are ahead in this episode!    Music in this episode is by Secret Agent 23 Skidoo from his upcoming album, Infinity Plus One.   Ask us a question, listen to more episodes, and donate at tumblepodcast.com. Subscribe to Tumble Science Podcast for Kids and review on iTunes. Follow on Facebook and Twitter. Thanks for listening!