On Bimetallic-coil Thermometers (i.e. the relatively inexpensive food thermometers with a spike on one end and an analog dial on the other):
This food thermometer senses temperature from its tip and up the stem for 2 to 2 1/2 inches. The resulting temperature is an average...If measuring the temperature of a thin food, such as a hamburger patty or boneless chicken breast, the probe should be inserted through the side of the food so that the entire sensing area is positioned through the center of the food.
Turns out that I've actually been doing that last part wrong for an entire year.
I'd just stick the tip of the thermometer into the food, and it would tell me a much lower temperature than what was actually going inside of that food. So then I'd cook the food in the oven for a completely unreasonable amount of time.
This mistake would be almost understandable, were it not for the fact that a few months ago I got a different thermometer, used the just-the-tip methodology once again, saw the low reading results, and thus concluded that the entire low-end food thermometer industry was peddling useless junk.
Now going to go eat dinner. I'm having stir fried broccoli, and chicken breasts that have been heated to an internal temperature that I now understand to be a toasty 182 degrees.--
See also the clip from The Office (U.K.), in which Gareth calls the manufacturer of his malfunctioning calculator.