Question

Why thermometers only mesures hotness of the body not?

Answer 1

The thermometers that we use at home and clinical use are known as clinical themometers.
Lab thermometers are used for experimental purpose and is not used for measuring body temperature.
Lab thermometers are designed to measure immediate temperature so upon removing it from a body orifice to read it the temperature reading will change.

Clinical thermometers are designed to read and hold the maximum temperature encountered.

That's why one supposedly could fake a fever by holding the thermometer on a radiator until it read 105F so you could go to the hospital instead of school.