Everyone agreed on the first step: Solve inside the parentheses, for 2+2=4. But after that, people split down two paths. Some multiplied first, while others divided, leading to different answers—1 and ...

Say you’re at a party with nine other people and everyone shakes everyone else’s hand exactly once. How many handshakes take place? This is the “handshake problem,” and it’s one of my favorites. As a ...