So I was fixing up a mountain bike at the local co-op. I'd swapped out a wheel with a bent rim, and noticed the old cogs were really worn in places. Then I noticed the chain wouldn't engage with more than 4 or 5 teeth of the crank at a time, and the same with the cogs on the replacement wheel. The chain had stretched a full quarter inch over 12". I didn't know that was even possible.