At some point in your teenage years, you probably kept a compass and straightedge in your backpack, learned the ways to prove two triangles are congruent, and knew what a secant was. It all had a taste of the classical about it: Euclid, Archimedes and Pythagoras had figured everything out and passed it down to us. But geometry may be more democratic than it seems. As a group of native Amazonians showed, you don't need to go to school to explain Euclid.
French researcher Veronique Izard and her colleagues wanted to know if an understanding of Euclidean geometry is intuitive. It makes sense for humans and other animals to have a basic sense of shapes and distances, so we can find reachable fruits and flee approaching predators. But our eyes often deceive us. So do children, or remote tribespeople, instinctively understand that two parallel lines never cross? Or how many points define a line?
