Listener Mail: Human Expansion, Tipping in America, and Shoutouts

The podcast discusses topics such as a touching letter from a grateful listener who credits the show for inspiring their upcoming book. They also explore disturbing photos, artistic techniques, and sponsored ads. Another topic is the role of intelligence in spreading species and exploring the cosmos. They investigate mitochondria and promote educational podcasts. Lastly, they discuss the expanding practice of tipping in the US and its impact on employees and consumers.