What is the difference between an oral thermometer and a rectal thermometer?

The tastes differ. An oral thermometer has a longer slender end that's inserted (into the mouth), compared to a rectal thermometer that has a short bulbous end inserted (into the rectum). The bulb reduces the chance of tearing the rectal wall, especially in infants. Some digital models have a setting for rectal vs. oral on the unit. The rectal temperature is known to be a little warmer and more accurate to body temperature. If you still have a glass and mercury thermometer, be aware that mercury is a deadly toxin, and consider replacing the thermometer with a digital model.