When a man's temperature is taken with a clinical thermometer, this is what happens. At first the difference in the temperatures of man and thermometer is considerable and the mercury expands with rapid heating. When the thermometer's temperature is near to that of the man's body, the heating of the thermometer takes place slowly and the mercury also expands slowly. Therefore a considerable time is required for the thermometer to be heated to the temperature of the man. When the thermometer is taken out there is a great difference between the temperatures of the thermometer and the surrounding air, the volume of the mercury contracts rapidly and it is enough to shake the thermometer for the column of mercury to occupy the space which is empty in the bulb.