How Does My Refrigerator Know When The Filter Needs Replaced?

Do our refrigerators have a water chemistry analyzer to detect when the filter is no longer making good water? Or perhaps it measure how many gallons goes through? With different parts of the country having different water qualities, how the hell does a big dumb thing like refrigerator have the smarts to know when a filter needs to be changed? If you have ever asked any of those questions, feel…