Why is there a need for us to shake the clinical thermometer prior to each use?

If the clinical thermometer is a Mercury based thermometer, it needs to be shaken down prior to each use. This is because there is a pinch point between the mercury reservoir bulb and the capillary tube, to ensure that the mercury does not shrink back into the bulb after the temperature has been taken. The mercury has therefore to be shaken back into the bulb before another measurement can be made.

One should note that in many countries use of mercury based clinical thermometers is now discouraged (because mercury is toxic) and electronic thermometers are used in stead - these do not need shaking.