Why do you flick thermometers before using them?

This is only in the case of of clinical thermometers, which usually have Mercury inside them. These thermometers have a constriction just after the bulbthat allows the mercury to flow due to the pressure of expansion and contraction but is thin enough not to allow the mercury back into the bulb, to allow people to read the thermometer without having to hurry. people flick the thermometers to force the mercury back into the bulb so that temperature can be taken again.