Closed ≠ Not Open: A Topologist's Nightmare

The teacher marked "closed" as the opposite of "open" and gave it a checkmark. Any normal person would move on, but mathematicians? They're twitching uncontrollably right now. In topology, a closed set and an open set aren't opposites at all—they can overlap or even be the same thing! A set can be closed, open, both, or neither. This is why mathematicians can't have nice things... or normal conversations at parties. The caption perfectly captures that moment when a mathematician spots this error and launches into an impromptu lecture that nobody asked for. Trust me, I've cleared entire rooms with discussions on non-Euclidean geometry.