Right. Before all water at the bottom has evaporated, the temperature is 100C. After all water has evaporated, the temperature rises uncontrolled due to lack of

Right. Before all water at the bottom has evaporated, the temperature is 100C. After all water has evaporated, the temperature rises uncontrolled due to lack of latent heat. So basically it just shuts off when the temperature exceeds 100C, and the classic mechanism is to formulate a magnet that loses its magnetism at that temperature (all permanent magnets have an operating temperature, and it can be tweaked during manufacturing) and at that point it's just a leaf spring switch that the magnet conditionally attracts.