Why is the thermometer keep under the tongue?

The internal body heat is present under the tongue. So, Thermometer is kept under the tongue to get the correct reading of inner body heat. This way we can measure the temperature of our whole body.

Explanation: -

In our boy, the rectum (part of the lower gastrointestinal tract) and the tongue are the most accessible areas of the body to measure the temperature.
The tongue is a more pleasant area to measure the temperature & it is kept below the tongue as it has a load of blood vessels on the inner side of the tongue.