Your exposed parts r always cooler than than ur covered body parts & internal areas That is why ur core temp ( which is either oral or sometime rectal temp) is considered to the exact measurement of fever . As other temp in other areas can be affected by environmental factors & core temp is affected by body functions only Important thing is u should not drink hot or cold liquids at least 1/2 hour prior of putting the thermometer in the mouth

Fever in the mouth and tongue could be due to an erroneous measurement of temperature after drinking a hot drink like coffee. If in doubt, check in the armpit for a full 2 minute or till the tinkle from an electronic thermometer is heard and then add 1 deg Fahrenheit to arrive at the correct temperature.

We have to see the core temperature of the body to find out fever. For that you should take under surface of tongue or arm pit in a regular thermometer. Right part of the forehead in Infrared Thermometer.