A neat puzzle from Alex Tabarrok, who is clearly in the top 50% of my ranking of MR bloggers:
During WWII, statistician Abraham Wald was asked to help the British decide where to add armor to their bombers. After analyzing the records, he recommended adding more armor to the places where there was no damage!
The RAF was initially confused. But just think about it and it makes sense. (Hint: Tabarrok’s title–which I copied above–is relevant.)