Let's start with a short story:
You are babysitting a particularly clever eight-year-old child. He complains that he is feeling feverish. It's your responsibility to take care of him while his parents are out, and you have no reasons to question his motives, so you find a thermometer in the bathroom and are planning to take his body temperature.
Do you:
a) Hand him the thermometer, show him how to use it, and tell him to come see you in 10 minutes?
or b) Stick the thermometer under his armpit and wait a while for the temperature reading.