Congestion happens when too many things try to move through a space that can't handle them all, causing everything to slow down or stop. When traffic congestion strikes a highway during rush hour, cars pack together bumper to bumper, creeping along instead of flowing smoothly. The road itself hasn't changed, but too many drivers trying to use it at once creates a jam.

Too much stuff gets heaped into too small a space. When a popular website experiences congestion, thousands of people trying to access it at the same moment overwhelm the servers. When your nose gets congested during a cold, swollen tissues and mucus clog the narrow passages that normally let air flow freely.

Congestion isn't about broken systems. It's about systems working at or beyond their capacity. A hallway handles normal traffic fine, but becomes congested when everyone rushes toward the cafeteria at once. Cities struggle with congestion because roads built for fewer cars now serve populations that have doubled or tripled. Understanding congestion helps explain why timing matters: leave for school ten minutes earlier, and you might avoid the congestion altogether.