In some semiconducting ceramics a gain in temperature causes grain boundaries to become insulating. This is most common in mixtures of heavy metal titanates. We can adjust the transition temperature (the temperature at which this happens) over a wide range by changing the chemistry. In this kind of material, current passes through the material until it reaches the transition temperature, at which point the circuit breaks and current flow stops. We use these ceramics as self-controlled heating elements in, for example, rear-window defrosting circuits in cars.