Anemometer

Real World Example

Imagine trying to gauge how crowded a party is just by listening to the noise level from outside the door. Just as the volume and intensity of the sounds help you estimate how many people are inside and how lively the party is, an anemometer uses the speed at which its cups spin to determine the wind's speed. The noise level at the party is like the wind speed: both are invisible forces that can be measured indirectly by their effects on other objects, like the spinning cups of an anemometer or the volume of chatter at a party.