According to the website HowStuffWorks.com, "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."
From the site HowEverythingWorks.org, "The electronic fever thermometers that you can buy in a grocery store use a thermistor to measure temperature. A thermistor is a semiconductor device that acts as a temperature-sensitive electric resistor. At very low temperatures, a thermistor is essentially an insulator—it has no mobile electric charges and thus can't carry electricity. But as its temperature increases, thermal energy rearranges the charges in the thermistor and it has more and more mobile electric charges. Its ability to conduct electricity increases with temperature fairly dramatically—it gradually becomes an electric conductor. The thermistor used in a fever thermometer is designed to undergo this rapid change in electric resistance at temperatures near 98° F. A simple computer inside the thermometer measures the thermistor's electric resistance and determines the thermistor's temperature. It then uses a liquid crystal-based display to show you what that temperature is."