(MayoClinic.com) If your child feels warm or seems under the weather, it's probably time to take his or her temperature. Sounds simple enough — but if you're new to it, you might have questions. Which type of thermometer is best? Are thermometer guidelines different for babies and older children? Here's what you need to know to take your child's temperature.

A glass mercury thermometer was once a staple in most medicine cabinets. Today, mercury thermometers are no longer recommended because they can break and allow mercury to vaporize and be inhaled. When choosing a thermometer, consider these options:

• Digital thermometers. These thermometers use electronic heat sensors to record body temperature. They can be used in the rectum (rectal), mouth (oral) or armpit (axillary). Armpit temperatures, however, are typically the least accurate of the three.
• Digital ear thermometers (tympanic membrane). These thermometers use an infrared ray to measure the temperature inside the ear canal. Keep in mind that earwax or a small, curved ear canal can interfere with the accuracy of an ear thermometer temperature.
• Digital pacifier thermometer. Your child simply sucks on the pacifier until the peak temperature is recorded.
• Temporal artery thermometers. These thermometers use an infrared scanner to measure the temperature of the temporal artery in the forehead.

Whatever type of thermometer you use, carefully read the instructions that come with it. Before and after each use, clean the tip of the thermometer with rubbing alcohol or soap and lukewarm water.

If you plan to use a digital thermometer to take a rectal temperature, get another digital thermometer for oral use. Label each thermometer, and don't use the same thermometer in both places.

For safety — and to make sure the thermometer stays in place — never leave your child unattended while you're taking his or her temperature.

The best type of thermometer — or the best place to insert the thermometer, in some cases — depends on your child's age.

• Birth to 3 months. Use a regular digital thermometer to take a rectal temperature. New research suggests that a temporal artery thermometer might also provide accurate readings in newborns.
• 3 months to 4 years. A rectal temperature provides the best readings for children up to age 3. In this age range you can also use a digital thermometer to take an armpit temperature, a temporal artery thermometer or a digital pacifier thermometer. However, wait until your baby is at least 6 months old to use a digital ear thermometer. If you use another type of thermometer to take a young child's temperature and you're in doubt about the results, take a rectal temperature.
• 4 years and older. By age 4, most kids can hold a digital thermometer under the tongue for the short time it takes to get a temperature reading. You can also use a regular digital thermometer to take an armpit temperature, a temporal artery thermometer or a digital ear thermometer.

• Rectal temperature. Turn on the digital thermometer and lubricate the tip of the thermometer with petroleum jelly. Lay your baby or child on his or her back, lift his or her thighs, and insert the lubricated thermometer 1/2 to 1 inch (1.3 to 2.5 centimeters) into the rectum. Stop if you feel any resistance. Hold the thermometer in place until the thermometer signals that it's done. Remove the thermometer and read the number.
• Oral temperature. Turn on the digital thermometer. Place the tip of the thermometer under your child's tongue and ask your child to keep his or her lips closed. Remove the thermometer when it signals that it's done and read the number. If your child has been eating or drinking, wait at least 30 minutes to take his or her temperature by mouth. If your child is too congested to breathe through his or her nose, you might need to use another method.
• Armpit temperature. Turn on the digital thermometer. When you place the thermometer under your child's armpit, make sure it touches skin — not clothing. Hold the thermometer tightly in place until the thermometer signals that it's done. Remove the thermometer and read the number.
• Ear temperature. Turn on the thermometer. Gently place the thermometer in your child's ear. Follow the directions that come with thermometer to ensure you insert the thermometer the proper distance into the ear canal. Hold the thermometer tightly in place until the thermometer signals that it's done. Remove the thermometer and read the number.
• Temporal artery temperature. Turn on the thermometer. Gently sweep the thermometer across your child's forehead. Remove the thermometer and read the number.

A fever is a common sign of illness, but that's not necessarily a bad thing. In fact, fevers seem to play a key role in fighting infections. If your child is older than age 1 and is drinking plenty of fluids, sleeping well and continuing to play, there's usually no reason to treat a fever.

If you want to give your child medication to treat a fever, stick to acetaminophen (Tylenol, others) until age 6 months. If your child is age 6 months or older, ibuprofen (Advil, Children's Motrin, others) is OK, too. Read the label carefully for proper dosage. Don't use aspirin to treat a fever in anyone age 18 years or younger.

Your child has a fever if he or she:

• Has a rectal, ear or temporal artery temperature of 100.4 F (38 C) or higher
• Has a pacifier or oral temperature of 100 F (37.8C) or higher
• Has an armpit temperature of 99 F (37.2 C) or higher

Keep in mind that an armpit temperature might not be accurate. If you're in doubt about an armpit temperature reading, use another method to confirm the results.

In general, contact your child's doctor if:

• Your child is younger than age 3 months and has a rectal temperature of 100.4 F (38 C) or higher.
• Your child is age 3 to 6 months and has a temperature up to 102 F (38.9 C) and seems unusually irritable, lethargic or uncomfortable, or has a temperature higher than 102 F (38.9 C).
• Your child is age 6 to 24 months and has a temperature higher than 102 F (38.9 C) that lasts longer than one day but shows no other symptoms. (If your child also has other symptoms, such as a cold, cough or diarrhea, you might call your child's doctor sooner based on their severity.)

When reporting a temperature to your child's doctor, be sure to give the reading and explain how the temperature was taken.

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