Why the thermometer kept under the tongue?

Because the tongue is a place in the human anatomy that has a very stable body temperature.

Other places in the anatomy that are use are, '1 the armpit, and '2 into the anus.

Modern technology has superceded a thermometer , for a 'ray-gun'. The 'ray-gun' is pointed at the forehead, and it reads the body temperature.

For a healthy human, the body temperature is 37.5 oC . This can range between 37 oC and 38 oC .

Outside these limits ;-

36 oC ; HYPOTHERMIC * Cold/Frostbite and liable to die)

39 oC ; HYPERTHERMIC * Hot/fever (convulsions)

Even when you exercise and feel hot, you body temperature will not be above 38 oC.