The Inspector's Sticker on the Grocery Scale
Have you ever watched a grocery worker weigh apples on a scale? You trust the scale to tell the truth. If it showed too much, you would pay for apples you never got. If it showed too little, the store would lose money. Either way, someone is cheated.
To keep things fair, many places send an inspector to check scales, gas pumps and other measuring tools. This person is often called a weights and measures inspector. The inspector carries a set of special test weights. Each one has been checked against a standard, which is an example that everyone agrees to trust. The inspector sets the weights on a scale to see whether it reads correctly.
Gas pumps get a similar test. The inspector pumps fuel into a container that holds an exact, known amount. If the pump says one amount and the container shows another, the pump needs repair.
When a scale or pump passes, the inspector puts a small sticker on it with a date. If it fails, the owner must fix it before using it again. Because of this simple check, shoppers can buy by weight or by the gallon and know the measure is fair.