How long should you keep the thermometer under your tongue?

You must hold your mouth closed and keep the thermometer in place for a certain period of time. If you have a digital thermometer, it will tell you when it's "done". If you are using an old "Mercury thermometer", the thermometer must stay under the tongue for at least a minute to 2 minutes.

NOTE: Mercury thermometers are being phased out. The mercury inside is highly toxic and you should dispose of all mercury thermometers by taking them to a recycling center that handles that. DO NOT THROW IT IN THE TRASH. Mercury is poisonous to every living thing.

UPDATE: GERATHERM offers MERCURY FREE thermometers that work the same as the mercury ones. They are supposed to be safe.