Measuring rectal temperature

Lay your child face down across your lap. Put a dab of petroleum jelly (sample brand name: Vaseline) on the end of the thermometer. Then, gently insert the thermometer into the child's anus until the silver tip is not visible (1/4 to 1/2 inch [6 to 12 millimeters] inside the anus). Hold the thermometer in place. A glass thermometer takes about 2 minutes. Most digital thermometers need less than 1 minute.