Why do human beings explore? And, why are the most adventurous explorers drawn to outer space? Naturalist and astronomer David Aguilar explains why the drive for adventure fades after childhood, and how we can regain it as adults. Also, a group of physicists dig into what the universe is made of. Janna Levin, Lisa Randall, and Lawrence Krauss debate black holes, and whether they actually exist. Their conversation is led by Ira Flatow, host of Public Radio International’s Science Friday.