Question: “Why don’t compasses point toward the nearest hospital because of the magnetic fields created by MRI machines?” (168).

MRI machines do influence compasses, but only within a small radius of approximately 10 meters. (Munroe notes how difficult it would be to “capture” an explorer using a net of MRI machines influencing their compass, although he also recounts that the navigational equipment on an emergency helicopter was recently disrupted by an MRI machine at the hospital where it landed.)

Question: “I noticed recently that the number of people in a family tree increases exponentially with each generation: I have 2 parents, 4 grandparents, 8 great-grandparents, and so on. Which got me thinking—are most people descended from the majority of Homo sapiens who have ever lived? If not, what fraction of all the people that have ever lived am I descended from?” (172).

You are probably descended from approximately 10% of all the humans who ever lived. Experts theorize that at an “identical ancestors point,” approximately 7,000 to 4,000 years ago, every person’s lineage either incudes every living human or has since died out. From that point backwards, everyone has an identical set of ancestors, but in the millennia since then, family trees have diverged.