Why can’t we use a laboratory thermometer to measure body temperature?

It is not convenient to use the laboratory thermometer to measure body temperature because the mercury level falls when taken out of the mouth.

Clinical thermometers are commonly used to measure human body temperature, and they typically contain a kink or constriction near the bulb.

The purpose of this kink is to prevent the mercury inside the thermometer from falling back down on its own once it has been heated by the body temperature. 

Without the kink, the mercury inside the thermometer would expand and rise when heated, but it would also fall back down when the thermometer is removed from the body resulting in an inaccurate reading of the body temperature. The kink or constriction in the clinical thermometer creates a small air gap, which acts as a barrier to prevent the mercury from falling back down on its own.