A fever is nothing to take casually in young children. If your child feels warm or seems under the weather, you probably should take his or her temperature. Sounds simple enough, but if you're new to it, you probably have a few questions. What kind of thermometer should you use? Should you take a baby's temperature the same way you take the temperature of a 5-year-old? What's best for your child?

If you grew up before the 1980s, your parents probably took your temperature with a glass mercury thermometer — a skinny tube containing the silvery liquid metal. Such thermometers are scarce these days because of the health and environmental concerns surrounding mercury.

If you still have an old mercury thermometer, recycle it appropriately during a household hazardous waste collection sponsored by your local health department. Your doctor may know of other ways to safely dispose of mercury thermometers.

Thermometers with newer designs are often easier to use than mercury thermometers. And, all the better for your child, they give you a quick result. Make sure you read the instructions that came with your thermometer, and never leave your child unattended while you are taking his or her temperature.

While ear thermometers and pacifier thermometers are designed for use in a specific part of your child, regular digital thermometers can be used in the mouth, under the arm or in the rectum. Here are some temperature-taking tips for these locations.

• Rectum. Lubricate the tip of the thermometer with petroleum jelly. Lay your child facedown across your lap, or on his or her side in the fetal position on the bed. Insert the thermometer no more than an inch into the rectum. Stop if you feel any resistance. Hold the thermometer in place until you hear the beeping signal.
• Mouth. Wait 20 or 30 minutes after your child finishes eating or drinking. Place the tip of the thermometer under the tongue, and ask your child to keep his or her lips closed and to breathe through the nose. Remove at beeping signal.
• Armpit (axillary). Remove your child's clothing above the waist and place the thermometer under your child's armpit. The thermometer must be touching skin, not clothing. It may help to hold your child sideways on your lap, thermometer arm closest to you, and then hug your child close until you hear the thermometer beep.

The best place to insert the thermometer depends on the child's age.

• Newborns. For babies less than 3 months old, start with an armpit (axillary) temperature. If it's higher than 99 F (37.2 C), take a second measurement rectally. Rectal temperatures higher than 100.4 F (38 C) in newborns and infants up to 3 months of age require immediate medical attention.
• 3 months old to 4 years old. For this age group, you can check your child's temperature rectally, or with an electronic pacifier thermometer or an ear thermometer.
• Older than 4 years. After age 4, most children are able to hold an oral digital thermometer under the tongue for the short time it takes to get a temperature reading.

A rectal temperature is the most accurate, although temperatures measured by mouth and ear are accurate if done properly. Armpit (axillary) temperatures are the least accurate, but are better than nothing.

Whatever the method, make sure you know exactly how to use your thermometer. Read the instructions that came with your thermometer. For safety — and to make sure the thermometer stays in place — never leave your child unattended while you are taking his or her temperature.

After the age of 3 months, children's normal body temperatures are higher than those of adults — primarily because young children have a greater surface area to body weight ratio and a higher metabolic rate than older children and adults. One study showed the rectal temperatures of healthy 18-month-olds were around 100 F (37.8 C).

A simple cold can sometimes cause a fever over 102 F (38.9 C) in young children. It would take a much more serious disease to produce that kind of fever in an adult.

Call your doctor if you have an infant younger than 3 months with a temperature of 100.4 F (38 C) or an older child with a fever higher than 104 F (40 C).

Most children tolerate a fever amazingly well. Low-grade fevers may actually help the body rid itself of viral infections. If your child is responding to you normally, drinking fluids and continuing to play, there's no reason to lower the temperature with medication.

On the other hand, if your child is listless, vomiting persistently, or complaining of a headache or stomachache, contact the doctor or go to an emergency department immediately. Also seek medical attention if your child's fever does not respond to fever-reducing medication or if a febrile seizure occurs.