Make a number the goal, and people find a way to hit it without doing the real thing.

Steps today10,000goal 10,000Goal reached!Their phoneOrange fitnesstracker, strappedto a bladeEach spin = one stepHasn't taken a step
Goodhart's law, in its popular form: when a measure becomes a target, it stops being a good measure. A step count was meant to measure walking. Once 10,000 steps is the goal, a fan can hit it for you. In this model the fan turns twice a second and each turn counts as one step.