system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When taking someone's temperature, why is the underside of the tongue a reliable spot?
gpt
It is because of the amount of blood flow close the the surface.  Same reason anal is also accurate and the arm pit can give a decent reading as well ( though not as accurate and anything internal).  That is the reason the ear is good too (using infrared to get to the inner ear)