It is now common to measure temperature with electronics. The most common sensor is a thermoresistor (or thermistor).
This device changes its resistance with changes in temperature. A computer or other circuit measures the resistance and converts it to a temperature, either to display it or to make decisions about turning something on or off. If you would like to build your own electronic thermometer, check out How Microcontrollers Work.
For more information on thermometers and related topics, check out the links below!