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When you think that your child has a fever, use a fever thermometer to take her temperature. There are three main types of thermometers--digital, mercury and plastic strip. The plastic strip type can give you an idea of whether your child has a fever, but it is not good for getting an exact reading. Mercury thermometers used to be the standard, but there is some concern about poisoning, so the American Academy of Pediatrics no longer recommends them. Different digital thermometers allow you to take temperatures from different places--in the mouth, rectum, ear or armpit.