Thermostat
How does it know it's cold?
It has one thermometer and one rule. Slide the room from cold to warm, set the limit, then warm the sensor with your hand.
Fool it
Senses
Rule
Does
For grown-ups: what's really inside
A thermostat measures temperature with a sensor (thermistor, bimetal strip, or digital probe) and runs a threshold rule: if the reading is below the setpoint, call for heat. This figure is heat-when-cold only — the same shape as a night-light's dark-threshold, with a different modality.
Simplified: your hand warms only the sensor. The room slider can still say cold while the machine thinks it is warm enough to shut the heat off.